^^ Unless you mean the magical kind...
I'm not actually sure which creatures are under-used in fantasy since I haven't read enough of the genre to judge, but there are plenty of creatures outside the obvious (dragons, werewolves, zombies, vampires) that I'd like to see more often.
Old fairy legends and ghost stories are a good source of inspiration. On the fairy front, the Dullahan and the Nuckelavee are both pretty awesome/scary. On the ghost front, I like the baykok and the Kuchisake-onna (the latter is quite well-known in Asia but AFAIK not so much in Western pop culture).
Although Lovecraft seems to be in vogue at the moment not all of his creations see as much use as others; moon-beasts, for instance, are pretty cool and not, to my knowledge, overused.
I'm quite fond of modern cryptids as well; a couple of my favourites are the ahool and the Mongolian death worm, but both of those have obvious parallels in fantasy fiction. There are a lot of others that don't, though. Older alien lore, in particular, has a lot of weird and wacky creatures that would be much more interesting to see than another retread of the greys, such as the Hopkinsville goblins and the Flatwoods monster.
I think the ahuizotl, the kamaitachi, the argopelter, the crocotta and the gulon are all interesting legendary creatures with relatively little fictional exposure.
edited 23rd Mar '12 3:11:06 PM by BobbyG