7. Leviathan
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Before becoming a summon spirit, Leviathan first appeared as a monster that swallowed the party back in Final Fantasy II. In Final Fantasy IV, he’s actually called the King of Eidolons and plays a big role in the game, using a giant tidal wave to decimate foes when summoned.
Since then, Leviathan has appeared as a summon in almost every Final Fantasy game. He may not be the most powerful summon around, but obtaining him and his tidal wave attack is always a big boost to your summoners repertoire.