Minotaurus

Minotaurus is one of the four Demon Guards in Impel Down. On its first impression it seemed to be like a joke character, a bipedal Holstein cow with a runny nose and pants. But in reality which relied on immense physical strength and speed in combination with its spiked mace

It is first seen beating two prisoners up for fighting, then locking them up in a cell for death row, then again beating Jinbei in Level 5. Although only its shadow is shown during those times, it is clear that it is a Minotaur that shows no fear. The fact that it could beat a strong character like Jinbei until he is covered in blood suggests how strong it is (though at the time, Jinbei was chained down).

Later as Luffy reaches the starvation hell of Impel Down, it gets the order to battle him as well as Mr. 2, who was supporting Luffy. However, as soon as it sent Mr. 2 flying with a single hit, Luffy recognized its true powers and immediately activated Gear Second and used his Jet Bazooka to send Minotaurus flying to save Mr. 2.

Although Luffy used one of his powerful techniques to defeat Minotaurus, it appeared later, hunting Buggy and Mr.3 (it landed on top of them when Luffy sent it flying). At one point they come to meet Luffy and Mr. 2. Buggy used his secret weapon, the Muggy Ball, to take Minotaurus down, but even after a huge explosion it was still conscious. Even when Mr. 2 used a strong kick to finish it off, it stood still but was in great pain. Only as Luffy and Mr. 3 combined their abilities to use the Gomu Gomu no Tonkachi Rifle, they could fully defeat Minotaurus.

Links

 * Minotaur - Wikipedia article on the mythical monster.
 * Holstein - Wikipedia article on the bovine breed of Minotaurus.