Well it's as they say I suppose. First you get good, then you get fast, then you get good fast. Beginning artists always take a very long time on their designs and drawings which is only logical; there's a lot to learn and every piece is a big step forward. If you create enough of such pieces, you will get better and better at making them as you learn. Then, when you've got pretty good, you will start applying the things you've learned automatically which means you'll work much faster. That is when you can start making decent money with your art because the effort/income ratio is getting better and better.
But first you have to improve. Expand your visual library; collect pictures of animals that interest you (as well as animals that don't, actually), study them, try to understand how they work and why. Draw them and create your own variations. All fantasy is based on reality in some way or form, you can't make up new things out of nothing. You need to make sure you are always collecting new material and learning new things. Good luck!