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 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 Keepers (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 the psychic influence of the ents in the grove they're raised in. (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 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 by a workgroup taking a chunk of living stone/ore and carving a skeleton from it. This can be done by a single dwarf, but it's considered somewhat pitiable; children raised by a workgroup are far more common. The process is both physical and deeply magical, as the dwarves impress their personality and experiences into the new dwarf as an instinctual part of the carving process. The "parents" then excrete a thick, clay-like substance from their reproductive organ which the skeleton is wrapped in, and then the "doll" is set in a warm, dark area to mature. Over the course of a few months, the clay gradually differentiates into flesh, blood, and organs, engulfing and integrating the stone skeleton, and forming into a young dwarf.
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 (which produce the "clay" for children), significant facial and body hair, and a generally "masculine" look as judged by humans.
The third race of my setting is the "humans", which is actually a large set of related species united by their physical commonalities. (Not unlike the elves, with their multiple ent species, or the dwarves, with their significant physical and mental differences caused by their rock makeup, but we human readers are more comfortable treating them homogenously while jealously defending differences between the races closer to us.) It's perhaps somewhat more reasonable to call this group "beast-people", but for ease of use I'll continue to use the word "humans".
All humans have skeletons of bone, and were crafted by the gods from base animal species. "Traditional" humans (like us) are just ape-people, no different in theory from boar-people, lizard-people, bird-people, etc. They're all warm-blooded, have two primary genders, sexually reproduce in pairs, have live birth, and nurse their young. (Yes, even bird-people and lizard-people.) All humans can interbreed regardless of base animal, with the child randomly inheriting the base animal of one of the parents; however, most of the time people stay within their species. New base animals are invented regularly, either by the gods or by human experimentation. The hybridization shows up in various fashions: some are just base-humanoid with some animal features (most monkey-people, some cat-people, etc, generally referred to as "humans"), some are more strongly animal-linked with animalistic heads (the "minotaur" style, occuring in many species, generally referred to as "orcs"), and even some half/half mixes (centaurs, mermaids, etc - no general term covering this group as a whole).
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.)