Well, that's a hard one.
It will be helpful to have more info, if not a picture, for which the problem is probably finding it again.
Does it have legs (so it walks), or does it glide as a flatworm, or does it swim somehow. Does it look insect or crustacean-like (arthropod), or worm-like with no legs(platy*=flatworm, nema*=roundworm, or hirudinea=leech), or does it look like a segmented worm with legs (polychaeta). In the absence of more info, my guess is that it is just a monster.
Good luck, Francisco.