Some time ago on Twitter, I riffed on the idea that bone was such a popular thing to do necromancy with - necromancers always seem to wear bone armor, use bone wands, animate monster skeletons, etc. Humans obviously are filled with bones, but what if other fantasy races weren't, and thought it was really weird/creepy that humans were? They'd still have skeletons, just made of something else - elves would have wood skeletons, dwarves would have stone skeletons, etc.
This came to mind again recently, and I spent more time mulling over the concept, and what it might mean. In my forever-developing fantasy world, I knew I wanted some "familiar" fantasy races, but also some weird riffs on them, and this seemed like a fun detail to build on. It turned out really great, actually!
When the gods created the intelligent races, they took from many sources. Humans and other beast-people were uplifted from animals, and still share most of their qualities - short lifespan, high aggression, etc. But the gods took inspiration from other sources as well. Elves were crafted from trees, dwarves from the earth. This is reflected in their unique biologies - they're mostly similar (all have blood, flesh, etc), but they have different cores, and reproduction methods.
Elves, or tree-kin, are descended from trees, and have skeletons of wood. Elves are actually a metamorphic species; the traditional slender, pointy-eared humanoid is their larval form. Maturity is not guaranteed, but the lucky few who reach it develop into living, intelligent trees known as Ents.
Ents are extremely long-lived (ten+ centuries at a minimum), and are the keepers of "elven" culture. They think slowly but wisely, have excellent memories, and are powerful spellcasters. (Their spellcasting is mostly geared toward sensing/seering, and geomancy, thus the perfect weather around elven groves.)
The massive ents - physically similar to redwoods to an untrained eye, but actually forming a number of distinct and varied species - regularly produce flesh "fruits" containing elf embryos. When these ripen, they're harvested by dryads (elves that are fully infested by the psychic influence of an ent, and act as their eyes, ears, and hands in the world), who help extract and raise the child. The children mature very quickly - a "late teens"-looking elf starting their adventuring career is actually about five years old. They reach mental maturity at the same time, due to spending most of their childhood in hypnogogic trances where their mind is shaped by the psychic influence of the ents in their grove. (An elf child raised outside a grove, if they even survived, would be considered severely developmentally challenged, as their mental development requires deep psychic learning from an ent.) A fresh, mature 5-year old elf, ready to take on the world, is already world-savvy and knows a trade or two, and will immediately start looking for exciting experiences.
Elves generally live to their early 30s, if they're not killed by something beforehand. Most elves simply die of old age then. A bare handful, those that have grown powerful and wise in their limited time, feel a pull to return to the groves and metamorphize into their ent form, creating a new tree.
Elves are completely sexless. The gods crafted them to look similar to the other intelligent species, but they don't have anything going on "down there" (smooth as a Barbie doll), and they don't develop breasts. It's common for an elf to pick up gender from the non-elves they hang out with, and so individual elves may refer to themselves as male or female and act/dress accordingly (but it's also not unusual for them to identify as neither). They typically have an "androgynous" appearance compared to other species. They're not necessarily asexual, tho - elves are often happy to engage in sexual activity with other races, and can engage in most forms of intercourse (they have hands, mouths, and butts, after all).
Elves are predisposed to danger and adventure; their high populations and short lifespans are intentionally geared to push them toward dangerous and exciting endeavors that will produce good ents. As a result, they're killed by the boatload by the monsters and hazards of the wilderness and dungeons. Elven bodies decay quickly and cleanly, and even their skeletons turn to mulch in a year or two. Elves don't have an inborn tendency to revere or mourn their dead, and will simply let them decay in a private place unless the surrounding culture has strong mores against it.
Elven culture is as two-sided as their biology. Young elves are often revolutionaries and firebrands, among the societies of other species. Among their own, tho, elves have little to no influence over the ents, whose typical lifespans are measured in millenia and dominate long-term elven culture.
Elves integrate into other societies reasonably well. They can form ordinary families with other species, but when other social forces aren't too strong, they are naturally prone to forming groups of a half dozen or so people. These groups are most akin to friendly roommate relationships, rather than the devoted polyamory of the dwarves; elves freely leave and rejoin as their life circumstances change, and happily group with humans (and dwarves that haven't yet found a workgroup).
Ents organize into groves, ranging from tiny young groves of a dozen to ancient ones containing hundreds or thousands of ents. They think very slowly, but very powerfully. Ents are intrinsically unsuitable as player characters - "playing" an Ent is probably best modeled by games like Civilization. This also provides good insight into their thought process - a single "turn" in a Civ game can cover 20 years of time, but only consists of a handful of broad-ranging strategy decisions, informed by distilled statistics about thousands or millions of individuals. You don't respond to local, immediate concerns, instead always planning for the next century or three. Many scholars among the shorter-lived races believe that history is mostly just a millenia-long struggle between different parts of Ent culture, using their skill at prophecy and their influence over their elven children to guide the future.
Having a kingdom near or containing an ent grove is a mixed bag. You have to accept constant waves of elves, with their revolutionary tendencies, and always live under the specter that your entire kingdom may be just a pawn in a long-term ent power play. On the plus side, the ent geomancy means your kingdom will always have beautiful weather and bountiful harvests, and their oracular guidance is extremely helpful (until it's your turn to be deposed for some private ent reason).
Dwarves, or stone-kin, are born of the earth, and have skeletons of stone. They are long-lived (average lifespan about 250 years) and tend to be stable, conservative folk. The type of stone they're made from has a significant effect on their abilities and disposition.
Dwarves are flesh and blood, like other mortal races, so they still need food and water, but they also naturally draw sustenance from merely being surrounded by living stone. A dwarf snug in a deep mine, with barely-worked stone all around, needs a hundredth of the food and water that they would need aboveground. Dwarves dwelling in the massive cavern-cities the race is famous for, tho, are usually only exposed to worked stone under their feet, and thus have roughly normal dietary requirements, requiring farming and trading. However, in case of famine, dwarves merely need to scatter and dig deep; it's lonely, but they can survive for years on almost nothing.
The dwarven love of ale is well-known, and is not just a stereotype. Alcohol tickles the dwarven palate the same way sugar does for humans; it's just intrinsically enjoyable. Furthermore, tho, alcohol keeps for a long time; even aboveground, wine or liquor can age for years, even when regularly opened to air for sampling, without going bad. A hearty dwarven ale, brewed strong and then fortified further with liquor, can stay unspoiled for months. This is very compatible with the lifestyle of a deep-digging dwarf - with the earth providing most of their living requirements, a single cask can fill in the rest of their food and water needs for months at a time. Note, tho, that dwarves are not intrinsically resistant to alcohol - most dwarves are granite-boned or basalt-boned, giving them the Tough trait that does indeed boost alcohol resistance, but other types of dwarves get drunk just as easily as most humans.
Dwarven families bond into polyamorous "workgroups". Dwarven relationships are extremely deep and almost always lifelong. (As a result, dwarves don't typically pair-bond with humans. The dwarven equivalent of a serial philanderer, worthy of pity and disgust for not being able to maintain a relationship for more than a decade or two, is actually a sturdy, dependable life partner for a human.) The poly relationships are typically a "complete graph", with every member bonded to every other member, and typically start with 3 members and grow to a dozen or so by the end of the group's life. (Some workgroups continue to take on younger members and thus persist for a long time; these "dynasties" are typically more fragmented graphs, with saturated subgraphs that are connected to each other along a few relationship lines.)
New dwarves are produced directly from the Stone and the Lifestream that circulates thru the earth. Without any intervention, new dwarves can spontaneously generate when an errant whisp of Lifestream peels off and settles into some stone; most Underground life is produced this way, with Dwarves being among the highest-intelligence ones so produced. These naturally-occurring dwarves still benefit from the collective unconscious carried by the Lifestream, meaning a lone natural dwarf can join up with a developed community and pick up their traditions without too much culture shock.
Within an active dwarven community, tho, most new dwarves are intentional. A workgroup performs religious rituals to coax the Lifestream to well up and suffuse a chosen bit of living Stone, and intermix their own souls into the developing soul energy, impressing their personality and experiences into the new dwarf. This process takes a month or two, at the end of which a new dwarf awakes.
A "newborn" dwarf is completely physically mature, the same size as an "adult" dwarf. Their skeletons do not grow over time. The major physical difference separating the young from the mature is their hair, particularly their beard - they're born bald as a cueball, tho most of their body hair develops in a few months, and a full adult beard takes nearly 20 years to grow. Other than that, young dwarves just need time to develop the strength and dexterity of their parents; they're ready to help digging and carving right away. Their mental preparation takes longer - newborns effectively have dissociative identity disorder (aka multiple personality disorder), as the different traits and impulses from their parents exert themselves independently. Young dwarves thus have somewhat erratic behavior, and have the tendency to parrot their parents speech and voices for a while. Over time, their minds integrate the disparate personalities into a cohesive and original identity, also taking about 20 years to complete. While a newborn dwarf could live independently, they'd be considered mentally handicapped, and without the guiding influence of their parents helping them reason thru their impulses, might not ever fully integrate their personalities.
The dwarven reproductive instinct is driven by a combination of biological need, to fill holes in the workgroup, and simple desire for children. Small, younger workgroups thus are more likely to have children, while the urge drops off as the workgroup "fills" and ages. However, dwarves never lose the ability to reproduce.
When a dwarf matures to adulthood, they naturally leave their birth workgroup. Dwarves have a much stronger and more pervasive anti-incest instinct than humans do; while the human instinct just discourages sexual activity with close kin, in dwarves it discourages even having close friendships between adult kin. So, after helping the workgroup for the two decades it takes them to mature, an adult dwarf leaves and travels to other workgroups, until they find a new workgroup that they end up life-bonding with.
Dwarven personalities are a mixture of the stone they're carved from, and the parents carving them. Stone is the most important, laying the "bedrock" (no pun intended) of their temperament. Most dwarves are granite or basalt-based, giving them a slow, sturdy mentality, capable of weathering hardships and making them physically tough. Other ores have various effects on the dwarven temperament: silver or gold in the ore makes them more garrulous and charismatic, natural leaders; sandstone gives them an affinity for the sea; many types of gem produce frail, but magically powerful, dwarves.
Because not all stone is suitable for child-carving, dwarven communities are often based around good veins. Large kingdoms develop around granite/etc deposits; the mental stability granted by those ores is a major source of the stereotypically conservative dwarven culture. More exotic ore veins are rare and jealously guarded by their discoverers, to ensure that the new family line based on the ore will extend for generations. As a result, there are always small ideosyncratic dwarven communites (sometimes as small as a single family) dotted across the countryside as well, surprising an unwary traveler expecting all dwarves to be the same. (In game terms, the stone you're carved from determines your Trait.)
The carving also has a major influence on the child. Thru carving, the parent(s) impart their personality, opinions, and experiences to the child. This enables them to begin life more fully-formed than humans, but also tends to limit how independent they can be; dwarven children do not stray far from the tree, as it were. This is the other major contributor to the meme of dwarven conservatism - between their long lives and the significant intrinsic influence of their parent(s), dwarven culture changes much more slowly than humans.
Dwarves are mono-sexed beings; any dwarf can produce children, and in any combination of dwarves. Physically, tho, they resemble male humans, including having penis-like genitalia, significant facial and body hair, and a generally "masculine" look as judged by humans.
The third race of my setting is the "humans", or human-kin/beast-kin. All human-kin have skeletons of bone, and were crafted by the gods from base animal species.
Human-kin are warm-blooded, have two primary genders, have live births and nurse their young, etc., like real-world humans, regardless of what their animal spirit is. (Bird-kin or lizard-kin don't lay eggs, for example.)
All human-kin can interbreed, regardless of their base animal spirit. Children receive a random base animal spirit from one of their parents, which can be different from kid to kid in the same couple. (No "hybrids".) Rarely, rather than inheriting a base animal from one's parents, a human can receive a random one from the surrounding nature spirits. This becomes more likely the less well-adapted the parents' base animals would be to the environment the child is conceived and gestated in. For example, if a community of polar-bear humans became refugees and resettled to a desert, within several generations a large fraction of them would be desert animals instead, and find the surroundings much more comfortable. When there's a significant climate-adaptation difference between the two parent's animal spirits, children are also more likely to inherit from the better-adapted parent (also furthering the adoption of desert animals in our polar-bear refugee community).
The animal spirit can come thru in varying amounts: it can be effectively nil, giving the traditional "human" look; it can be moderate, giving a beast-folk or "metahuman" look with tufts of fur, animals ears, claws, etc; or it can be major, resulting in full animal heads (like minotaurs, or rakshasa, etc), full-body fur, etc, commonly referred to as "orcs". The degree of animal expression is passed on genetically, usually being an average of the parent's expression with a random factor added in, like height. (So two "humans" having children will usually have "human" children as well.) Orcishness can also result in other types of radical bodyplans, like mermaids or centaurs, but typically this kind of major bodyplan change is inherited from the mother.
Human Traits are generally understood to be determined by the stars you are born under, but are influenced by your base animal as well. (Most "orc" species have a higher percentage of the Strong and Tough traits, for instance, regardless of the time of year.)