In my setting, the world is Earth 50 million years in the future. Magic and the Spirit World exists, whatever, not worrying about explaining them, but I do look at where certain things come from in more detail. Dwarves evolved from badgers. Halflings evolved from spider monkeys. Orcs evolved from boars. Goblinoids evolved from rodents. Humans evolved into Angels (those that embraced cybernetics and lost sexual differentiation and redundant organs) and Devils (those that embraced genetic engineering), which explains why Aasimar and Tieflings are possible. Humans in the setting were a science experiment of the Angels to study their ancestry; giants are related to that experimentation (ogres are half-giants, the elemental giants are from those who were messed with by the elemental gods).
Birds evolved into the Drakes, which is a new Class of animals that includes hipogriffs and griffins, as well as wyverns, dragons, and my Valkyrie race (I've been using modified Aarakocra stats). Somewhere along the way (probably from human muddling), the first drakes evolved to be septapods. Griffins and Hippogriffs are more primitive members of the class, while others grew larger and reverted to being more reptilian. Other six-limbed creatures like basilisks are probably drakes as well.
Werecreatures and my beastial races are related to human genetic engineering as well. Like Moreau's, anthropomorphic animals were created through human genetic engineering. The first generations of these creations were done prenatally, and spawned species that breed true (and are close enough to humans that there are hybrids). Later generations of the experimentation tried using retroviruses to afflict adult subjects with the changes. This spawned lycanthropy. The varieties of beastial races and werecreatures are leopard, wolf, bat, rat, bear, and dolphin.
All the existing Elf races, including the half-elf, are varieties of half-elf. Currently, true "elves" are not allowed as player characters, and I tend to portray them as haughty beings of the spirit world. I'm working on stats for young elves, possibly taking the place of Gnomes in my setting, to be a player race. Adult elves are akin to gods, basically.
Lots of animals haven't survived the 50 million years, especially large mammals. New animals have taken their place, though I'm largely just using the stats of others. Cats, Dogs, and Bears niches have been filled by rodents, mustelids, and other previously smaller mammals. This is mostly cosmetic, and we just call them wolves and such for simplicity at the table. I let my more interested players pitch in for animal ideas, and sometimes we stat new animals because of it (we have a variety of flightless bats, whose wings have become long fingers for climbing and grabbing, who are blind but use echolocation to see and sonic attacks to disorient their prey).
The setting leans heavily into animistic religions. "God" is a term for benevolent spirits that people revere to gain a blessing, while "Demon" is a term for malevolent spirits that people revere to stay (or direct) their destruction. Druids are an older faith that focus on nature spirits, while Clerics are a newer faith and focus more on ancestral spirits. Everything has a spirit, from all creatures, to rocks, rivers, plants, and mountains. The power of a spirit is proportional to the size of it's physical form and its reverence; the spirit of a mountain is automatically more powerful than the spirit of a pebble, but the spirit of the sling stone that slew a giant and is a relic in a church is more powerful than a random pebble from a creek. Not all powerful spirits are sentient, not all weak spirits aren't sentient.
