 Spira also features various animal species and fictional species , such as the gigantic <unk> and the chocobo that are used primarily for transport purposes . Most other unusual creatures encountered in Final Fantasy X are " fiends " , monsters created from the restless dead by Pyreflies to devour the living . Aeons and the unsent are also forms created by pyreflies . Sin , the bringer of destruction , is a powerful fiend that is made of high density pyreflies ; it can control gravitation forces to replenish its strength and even fly . Sin 's high concentration of pyreflies effect the pyreflies present in the bodies of those present , it is known as " Sin 's toxin . " Despite the Final Summoning , the core of Sin , is not destroyed and Sin " reborn " after a time . 
