it could be nearly any monster as far as i know: Here is a list of monsters that it could be in my eyes:
(These are taken from ffcompendium.com)
Abadon
Adamantoise
Ahriman
Antlion
Bandersnatch
Basilisk
Behemoth
Biblos/Apanda
Bomb
Cactuar
Catoblepas
Cerberus
Chimera
Cockatrice
Coeurl
Death Claw
Death Gaze
Demon Wall
Dual Horn
Flan
Funguar
Garuda
Goblin
Golem
Hecteyes
Imp
Iron Giant
Lamia
Lich
Magic Pot
Malboro
Mind Flayer
Minotaur
Mover
Mu
Ochu
Sahagin
Tiamat
Tonberry
Typhon/Chupon
Worm
Zuu

Or i just suck at riddles, which is also true.