How Do You Unlock Behemut?

[GTX]

So the question has pretty much been answered then.

Bahamut's data (i.e. name in summon list) was in KH, but no model was ever created for him, so when you summon him, the game will freeze.

FYI Bahamut - Final Fantasy Wiki

Bahamut is one of the most popular summons in the Final Fantasy series. He has appeared in several installments, as well as his own game, Bahamut Lagoon. He also appeared as a boss in Super Mario RPG: Legend of the Seven Stars. His attack, Mega Flare, sometimes translated as Sun Flare, deals non-elemental magic damage which ignores defense and evasion. He is typically considered the strongest among storyline summons and more often than not, the player must prove his/her worth by winning either against Bahamut himself, or, rarely, another storyline boss.