Leviathan

Leviathan is a summon that appears in most games in the series. It is a sea serpent, and its attack, Tidal Wave (also known as Tsunami) deals water-Elemental damage to all opponents. In Final Fantasy VI, it was replaced with the Esper Bismark.

Final Fantasy IV
Leviathan plays a major role in this game. He is the king of the Summoned Monsters that live on a parallel plane of existance in the Underworld. He knew that Rydia of Mist was one of the last surviving Summoners, and that she was potentially the most powerful. As a result, when Rydia was travelling with Cecil, Edward, and Yang to Baron on a ship, Leviathan attacked the ship and sucked Rydia into the Land of Monsters. He is later obtained as a summon once Cecil and his friends visit him in his home. They must defeat and obtain Asura in order to even talk to Leviathan. After obtaining Asura, Leviathan will challenge the party to a fight. If the party wins, they obtain Leviathan.

Final Fantasy V
Leviathan is a Level 5 Summon that guards Istory Falls. He protects the ancient tablet that lies there, and the party must fight him to get both the tablet and him.

Final Fantasy VII
The Leviathan Materia is found in Wutai after Yuffie defeats Godo.

Final Fantasy VIII
The Guardian Force Leviathan can be Drawn from Garden Master NORG.

Final Fantasy IX
The Eidolon Leviathan is obtained automatically at the Iifa Tree. It is taught through the Aquamarine. Only Princess Garnet can summon it.