CREEP Gigas Worm
Creeps are found in packs together or with Ogres. They're only found near the beginning of the game.
When you still have a weak party and fight one or two of these a shot of FIRE is handy. When you find them with ogres it's probably best to kill them off first with a shot of FIR2.
Creep is Gigas Worm in the Japanese version and FF Origins. I guess that means giant worm. (Learn more about Gigas on the Giant page.) I assume Square just made this thing up, because I haven't a clue where it could have come from. I'm pretty sure they don't come back in any later games either.