Behemoth

The Behemoth (ベヒーモス) is a powerful Heartless which appears in Kingdom Hearts. It is based on Behemoths from the Final Fantasy series, large purple monsters with two bull-like horns and a lion-like body. It is one of three powerful Heartless released by the Final Keyhole, which is also the Keyhole to Hollow Bastion. The other two are Phantom and Kurt Zisa, who leave to attack Neverland and Agrabah while Behemoth remains and guards the keyhole.

Although the first Behemoth is found in Hollow Bastion, other Behemoths are found in the Olympus Coliseum and End of the World. These other Behemoths were repurposed as Destroyed and Arch Behemoths in Kingdom Hearts Final Mix.

Kingdom Hearts
''Immensely powerful Heartless. With their large frame, they trample enemies, and repel most attacks. Damaging their weak point makes them lose consciousness. Strike them with potent attacks while they are down.''

Kingdom Hearts
Despite their menacing appearance, Behemoth can be defeated relatively easy, compared to many other bosses. His only weakness is the small black horn on his head, which is more often than not out of the player's reach while standing on the ground. The trick is to jump on to the thigh of one of his hind legs and then further on to his back (or just directly onto his back, if possible), and then simply run up to the back of the horn to avoid falling off due to Behemoth's head movements. From this position, one can attack freely, and without the possibility of being attacked by Behemoth.

Also, Blizzard, Thunder, and Fire magic doesn't affect him. Also, when he charges up his energy ball rain attack (an orange ball will start glowing in between his horns) get under his back legs and stay there until the balls stop raining and then make a dash for the weak horn.

Your prize for winning in Hollow Bastion and the first battle in End of the World is the Omega Arts accessory. Another appears in the Hades Cup, where victory over it earns you the Blizzaga upgrade, and a fourth appears in the area just prior to the Final Rest save point in End of the World.