Not all creatures are created equally, and not all of them are nice or kind. There are 7 things you need to think about in essence of what you need to make a creature. Environment - Would not make sense to have a aquatic being on a mountain that has no water on it. Ecosystem - Does this actually have a niche in the area? General shape - This is important because it defines what it can and cannot do physically. Does it have any special powers, and if so, how does it work? Gravity - Denser materials will always weigh and be stronger then lighter ones. Creature type - Insect, bird, mammal, mythical.. etc... What does this thing eat? Another few thing to take into consideration for extra details: Is it predatory, and to what? is it social, or is it a loner? How would this thing see? There are many types of eyes out there. Does this thing have the same type of blood as us, or is there a chemical difference? How tough is it's hide? is it aggressive or passive? Is it smart? What is it's interests? Does this thing have a specific biome it prefers? How dangerous is it?