In the process of my current world build(s), I’ve been considering the questions of creatures and races. A few questions come to mind with both subjects. First, how many races? That one, I plan to address in greater detail later, so we’ll pass for now.
With creatures, that is beasts, animals, in fantasy and sci-fi we have to ask a few questions too.
Do we use Earth creatures?
This is the easiest way to discuss creatures. Everyone is familiar with the animals on Earth, or can easily become familiar with them. Assuming real Earth animals are used. Earth’s mythic and legendary beasts are also possible and familiar to many, of course, depending on how obscure the source the writer uses is. For instance, most readers are familiar with dragons and gryphons; but they are not familiar with delgeth and amarok. On the other hand, using Earth beasts on a secondary world can be problematic for suspension of disbelief. That said, fantasy has a tradition of accepting certain Earth beasts as standard (ex. horses, bears, rabbits, cats, dogs).
Original beasts, on the third hand, require a lot more work. In each case, the original creation needs appearance, ecology, feeding habits, mating habits, even psychology, depending on their role in the story. They can add a few layers of reality to the world, even a layer of uniqueness. Consider Rowling’s thestrals and pygmy puffs (even in an alternate primary world).
The other major, really important, question to ask is: where is the line between creature/beast and race?
This is a question that people have been asking since at least ancient Greece, perhaps even as far back as the prehistoric era. Numerous attempts have been made to answer the question. Reason was posited. Intellect was posited. The presence of a soul was posited. All were problematic. Certain beings also problematize the divide, such as werebeasts (ex. werewolves, werebears, werecows).
Personally, I’m not even all that sure where I place the dividing line. Intellect, possibly, but maybe not. Sentience (ability to feel), not as such. Creation of societies, maybe, but probably not. Communication, definitely not. I don’t think my decision to say, for instance, that pegasi are beasts and elves are a race is arbitrary, but I can’t exactly say why they are without finding flaws in the argument.
Other writers have this issue as well. Rowling is a great example. She writes of werewolves, for instance, as people, but includes them in a book of magical beasts. Likewise, her merfolk and centaurs demand to be classified by the magical government as beasts, not “people”, because they don’t want to be associated with humans. On the other hand, no one questions the place of house elves and goblins as “people”, even if they treat house elves as little better than beasts.