The birds in the sky, the mole in the ground, the tiger in the trees, today's prompt is answering what animals we could expect to see in your world. Some creatures simply serve to fill out the niches in the biomes in your world, providing meat for humans sometimes, but not being too notable otherwise. Maybe your world has a great importance placed on the animals in your world, where some are threatening beasts, or maybe they have a magical or corrupted origin that attaches them to the cosmology of the world. They may be a real obstacle in the world as entities who are relentless and are not susceptible to negotiation.
I'll say this a lot, some of us could spend a whole month on this subject, especially if you're filling out a monster manual for your world full of fiends and dire whatevers. Even if the creatures are not all that important, they can be used to great effect to convey how alien a world is. If a wizard strokes a small pet dragon who remains the size of a house cat, and it's not seen as any big deal, it drives home that these characters are comfortable in a world that is not our own.
If you have a huge roster of creatures to design, I can help at least start the ball rolling today. A great tip is to understand the biomes of your world, and the families that these creatures come from, and divide those up. You may have an entire monster family of devils, then an entire section of varied harpy families, your seas may be ruled by aquatic dragons who feast on various jellyfish who infest the sea. If you at least figure that you need to explore the different forms of these dragons and jellyfish, then you can work back from there. You can even figure out "What sort of places would these creatures inhabit?" And work backwards from there. If you have magical ruins, then you can think about what would be attracted by these magical energies or be left over. Maybe these ruins were abandoned long ago due to a great summoning circle gone wrong and creatures from another plane now inhabit the overgrown building and at the heart within the circle it was brought from this world from, a great beast remains, controlling minions and planning to reopen the gate to its own world or to find a much more reliable source of humans to eat.
Otherwise think of the food chain as well when creating a believable ecosystem. Which creatures are apex predators, which creatures graze upon the plants and run from the apex predators, which creatures have found a way to totally subvert the food chain and have found an obscure niche to fill licking rocks for nourishment.
You can also think about domesticated creatures. Humanity has had a long history of bonding with animals who serve a purpose. Cats eat rodents, dogs can help hunt alongside humans, and both provide companionship. Maybe races in your world have more aesthetic uses like tropical fish being cared for in a tank.
Now go wild and…
GET BUILDING!