Bahamut

Bahamut is one of the best known summons in the Final Fantasy  series, appearing in almost every major game to date. He's so popular that he even managed to appear in Square developed Super Mario RPG: Legend of the Seven Stars as a boss. The character also appeared in his very own Super Famicom video game known as Bahamut Lagoon, a tactical RPG that was unfortunately never released outside of Japan. In the video games, the player usually needs to battle Bahamut in order to make him your summon, though occasionally you'll have to battle another boss.

NES appearances
In the original Final Fantasy for the NES, Bahamut was known as the King of the Dragons. Here he would appear inside of a cave and upgrade the player's class if they brought him back a special token. In this video game, Bahamut did not appear as a summoned creature.

In Final Fantasy III, the group of players come across Bahamut and must flee the unbeatable battle. Once they enter Bahamut's Lair, they have to battle him without invincibility. After being defeated, Bahamut will offer himself as a summon, marking the creature's first appearance as a summon in the series.

SNES appearances
Bahamut appeared in all three of the major Final Fantasy video games that appeared on the SNES. In his first appearance on the console in Final Fantasy IV, Bahamut awaited Rydia, a summoner, to come and battle him so that he could become a summon for the group. She and the rest of the characters did so, and following this Bahamut claimed that she was the first summoner that Bahamut ever accepted.

In Final Fantasy V, the clan must once again climb to the top of a mountain and battle Bahamut in order for him to become one of the players' summons. In the game he is a level 5 summon. Finally in Final Fantasy VI, the player will receive Bahamut after defeating Deathgaze under certain conditions.