Why create creatures? Aren't there enough real world animals out there? Well yes, there are plenty of real world animals out there that can be used in a story, but do they really fit?
I mean, come on, do you really expect to find a normal everyday horse on an alien planet or alternate universe. Or all the same kinds of birds that exist in real life? Are they really going to look/act/be the same?
Yes, if your story is set on a realistic Earth or human space colonization (even here you can create creatures for the alien worlds) you have a virtual cornucopia of creatures to draw from both scientifically proven and mythical. But, if it's not, then you have the opportunity to stretch your imagination beyond the everyday and create creatures to fill in the gaps.
What you have to remember when creating a world is that it is your world, and can be anything and everything you want it to be, and have whatever type of creature you want to have in it. Not just the same everyday animals that everyone knows. Creating your own creatures is a fun and imaginative exercise.
Before I go any further I want to tell you that for all intents and purposes you will probably not create anything that hasn't been imagined before. Most all creatures created by artists or writers are inspired by something. Whether it is from something someone else has done before them or from something that occurred in real life. Even the creatures of myths and legends were based on some factual evidence. (Dragon bones in China are fossilized dinosaur bones, the Greek cyclops is supposedly based on a mammoth skull.)
That said, don't think that you can't create something uniquely your own. It's all dependent on how you treat and describe your creature. Perhaps it has a unique power, or an extra set of limbs. Perhaps it's identical to an existing creature, but the story around it is completely different.
Ultimately, your creature becomes yours when you add that little bit of flair to it that no one else has.