From chulls to owlbears, wargs to thestrals, fantasy has taken to the creation of its own wildlife. They make a world feel separate from the Earth we know, even if they purport to exist on Earth (beyond the reach of common knowledge). They can be man-eaters, beasts of burden, or simply animals that live out in the wilds going about their business. If you want to add imaginary creatures to your world, what do you need to know?
What do they look like?
Go beyond the limits of animals that might exist on Earth. If you describe a large, shaggy mammal with claws and an elongated snout, you might conjure the image of a bear in a reader’s mind. Focus on the differences from what people might be familiar with. Take that same shaggy mammal and remember to mention the spiked tail, the six legs, and the forked tongue that flicks as it hunts. If you don’t want to diverge that far from typical animals, consider why you’re not just using real-life creatures. Plenty of fantasy stories get by without invented creatures. If you’re going to create your own, do it with gusto.
How does a creature with six mouths eat? How many of them does it need to survive? If you have an animal that sucks the brain from its prey, how does it go about the extraction? If something is made out of crystal, what does it do with food? Creatures with purely magical anatomy are less relatable than ones that function on anatomical principles people understand. However, if something is going to simply be a monster, maybe “magic does it” is fine. Avoid examining these creatures too closely, lest you shine a light on a weakness in your worldbuilding.
How does it fit into the world?
So, you’ve got a beast in the wilderness that can eat men whole and has an insatiable appetite. Why hasn’t it either devoured whole swaths of civilization or starved to death yet? If pixies devour any corn that gets planted in the Field of Sileen, why do the farmers there keep planting it?
Also consider the displacement of normal animals. If your people ride six-legged lizards and have spiders carry their burdens, what happens to horses and oxen? If there is a breed of especially tasty, low-maintenance vermin, what would become of chickens?
How does it fit into the story?
It’s always a good idea to focus on the story in your worldbuilding. If you include too many extraneous details about your invented world, you risk distraction from the story you’re trying to tell in it. What would happen if you weren’t to use these new creatures in your world? Do the ones you create do a better job fitting your needs than the ones you can find at the local zoo? It’s valid to use a creature of your own making because it’s more dangerous than a real creature (and thus, more of a challenge for a would-be hero), but many of the real ones are pretty deadly in the right circumstances.