Bahamut

Bahamut (バハムート) is one of Xigbar's weapons in Kingdom Hearts 358/2 Days. It can be upgraded to Bahamut+ and Bahamut++.

It is named after Bahamut, one of the  Final Fantasy series's most famous summons, a great and powerful dragon appearing in all of the main Final Fantasy games, except  Final Fantasy II and  Final Fantasy XII. Its signature attack, Mega Flare, appeared in the form of an extremely powerful Fire-based Sleight in Kingdom Hearts: Chain of Memories and Kingdom Hearts Re:Chain of Memories.

Bahamut was originally planned to appear as a Summon in  Kingdom Hearts, but was removed before its release. Through the use of cheat codes, it can appear in the list of summons in the battle menu; however, selecting it from the list will cause the game to freeze.

Oddly, Xigbar seems to have a few connections to Bahamut. One of the two main games in the Final Fantasy series that Bahamut doesn't appear in is  Final Fantasy II, which matches Xigbar's rank in Organization XIII.