When I invent creatures for my fantasy world, I use a few techniques to brainstorm ideas and polish them. I usually use the this one to polish ideas, but it can also be used to brainstorm new ones. Either way, one way to worldbuild new creatures for your world is to Fit the Niche.
As in, where in the world does the creature fit? Where does it live, what does it eat, and at what time of day? Does it hide in trees, or in the ground, or perhaps not at all? I like to carve out a niche for creatures I invent, finding reasons for why and how they would exist. As a biologist, I like my creatures to make sense, and am delighted when strange creatures find a special place in their imagined environment.