Never hesitate to create your own creatures, sometimes that can be the most fun part of the whole thing.  Think of it like aliens:  they almost never have any prior art to draw on. 
As far as tweeking the less common things, you can always put a new spin on something old to make it your own.  This has been done time and again with Vampires of every shape and size, for example.  Personally I have a soft spot for unusual were-creatures, myself.  One of my perpetual WiP's opens with the MC being attacked by a group of Were-ants, for example.  It's a Queen with the actual supernatural power, who thrall's and empowers random human bruisers, giving them super strength (in line with the Ant's proportional strength) and the group coordination of a Hive Mind.