The hard part of fantasy creature design is learning the anatomy & biomechanics of a wide variety of animals so that when you make weird composites or freestyle entirely they really feel plausible and functional.
The harder part of creature design is learning about animals more obscure than a rhinocerous because on the one hand you have to have what it takes to go looking for strange animals, but on the other hand if you manage it you can skip a stupendous amount of the stuff I mentioned above by stealing This Thing wholesale and bejazzling it a little. Because the number of people who will call you out on "that's just a baby antlion" (cn horror movie plots, photos of bugs) is simply very small.