What I do if I want to create masses of ideas following a given theme, is to formulate some kind of idea generator and see what it makes by throwing random permutations at it.
So for fantasy creatures... hmmm...
Maybe, choose familiar creature and add a new feature of some sort. Also think of introducing a colour, or a material, or an element. Use a thesaurus to help find words which merge well together, and don't be afraid to use foreign languages too.
Dog + Fire = Flamehound
Cat + Cold = Frostleopard
As you see you can get some superb creature names
You could also think of the Greek mythological creatures, and how they are composed of two animals, like the centaurs. Try to think of some similar combinations, particularly merging human + animal...
horse + human = centaur
cow + human = tauren
tree + human = treant
rat + human = kobold?
amphibian + human = murloc?
If you want to stray away from humanoid, then move to life farther away... like germs.
Look at some photos of what these lifeforms look like, and think of how to adapt them for the environment they'll be living in: do they swim, or fly, or walk? Is their weight upright (few limbs) or down low (several limbs). Swim like a fish with a wriggly tail or flippers, or swims like a Jellyfish? If it swims it needs to be hydrodynamic (fairly smooth and streamlined). If it flies it needs to be aerodynamic, and have some sort of wing(s).
Intermediates like flighty rodents, have limbs as well as wing-like skin flaps. Amphibians have limbs, yet have moderately streamlined bodies, and the limbs double up as flippers of sorts. Swimming/diving birds do all environments, as their have large flat feet to stand on, to paddle with, and wings which fold flat tight against the body so they don't interfere with swimming.
Perhaps you can design your creature for the environment it lives in. It's another way of coming up with ideas.