Species

What could stop you from making a new species of mammal. You could have a creature with the face of a lion, ears of a bat, and a shape of the head as a mouse. you could make anything reality. Do not think that everything is limited or what your thinking is too extreme. If you want to believe a new type of creature is good in your story or imagination then do it.
Anything can be created in your mind, and I believe what you think in your mind will stick better to someone who wants to read sci-fi. What are you going to remember more a vampire who emotionally depress or a weird lion-bat-mouse combo creature?
Though sometimes people do not change enough, they give a same plot but different names. I believe what ever comes to your mind first you stick with it.
If you want a rabbit who has the abilty to breath fire then do it. I think i would be more scared of that then a real dragon, atleast I can hide from a dragon but the bunny is smaller than me.
If you are going to make a new species do not do what i just did make up a character in three seconds. If you are going to make it research it. Every writer had to research something to make sure what they are describing or telling is what it would look like.
You have to imagine, but you have to be realistic.