Without knowing the full story or what you are using these new creatures for I might suggest trying to use creatures that are already there if possible. Instead of creating new creatures trying adding different aspects to the ones you already created that will help with the current problem. Say you have a centaur that needs to cross a deep river but he can't swim so you have him call up a giant turtle of the deep to take him across.
If you don't want to bring in the turtle because you think you have too many creatures already then just use one of the centaurs own abilities. Such as they are excellent builders and archers, so most stories go, have him build a bridge using these abilities and his surroundings. Then not alone have you avoided bringing the turtle into the story but you have enhanced the character of the centaur.
You can so the same thing with non-important creatures in a story as well. Instead of say adding giant tree eating beetles when your character enters the dark forest, have a creature you have already introduced be following them. This creature could attack them while in the forest adding dimension to your story again without actually having to bring in new creatures. As always just my thoughts hope they help. D.