Leviathan

Leviathan is a summon in the Final Fantasy series. Its sea monster like appearance invokes its primary element - water. It initially appeared in Final Fantasy II for the NES, in which it itself was a mini dungeon rather than a summon. In the game it was known for engulfing ships and eating those that were aboard. Some, however, managed to survive, such as the Dragoon known as Ricard.

Its first appearance as a summon was in Final Fantasy III for the Famicom (note: this game wasn't released in America until the Nintendo DS remake). Its most potent attack in the game was Demon Eye, which would kill your opponent instantly unless it has immunity against such an attack.

In Final Fantasy IV for the SNES, Leviathan is once again a summoned beast, though his backstory is explained a bit more - apparently he is the lord of the summoned creatures that live in a parallel underworld. He knew of Rydia, and how she was one of the last remaining Summoners. Because of this, he awaits the playable characters' ship to near him, in which he engulfs the young girl and takes her to his own world. The group eventually find where she is, though to approach the summon they first had to battle the summon known as Asura. After this, Leviathan offered to battle them, and if they won the sea monster would join their party. In the beginning cutscene of the Nintendo DS remake, and adult Rydia is seen summoning Leviathan.

In Final Fantasy V, the players, in order to obtain the creature as a summon, must battle him first. This is, however, optional. In Final Fantasy VI, Leviathan appears exclusively in the Game Boy Advance remake. He would attack your boat in the game, and the only way to obtain him was to defeat the beast.

Other Nintendo system video games that Leviathan has appeared in include Final Fantasy XII: Revenant Wings, in which the summon has legs, and Final Fantasy Fables: Chocobo Tales.