Minotaur

The Minotaur (Μινώταυρος in Ancient Greek) was a creature with the head of a bull on the body of a man. He can't speak and is rather dull headed. However he does have strength and is angered easily. Like all monsters, the Minotaur can't be fully killed, and it will just keep coming back.

History
The Minotaur was born of a human, Pasiphaë, and a white bull. Poseidon sent the white bull to King Minos for him to sacrifice in his honor, but King Minos did not sacrifice the bull, so Poseidon cursed his wife to long for the bull. She had Daedalus make a fake cow for her to hide inside and lure it.

The result of their union was the Minotaur.

Pasiphaë nursed him in his infancy, but he grew and became ferocious. Minos, after getting advice from the Oracle at Delphi, had Daedalus construct a gigantic maze to hold the Minotaur. Its location was near Minos' palace in Knossos.

He dwelt at the center of the Cretan Labyrinth, which was an elaborate maze-like construction built for King Minos of Crete and designed by the architect Daedalus and his son Icarus who were ordered to build it to hold the Minotaur. The Minotaur was eventually killed by Theseus, the son of Aegeas with the help of Ariadne.

The Lightning Thief
In the first book, The Lightning Thief, Percy Jackson is running to Camp Half-Blood when Minotaur attacks. He is described as having fur from about the belly-button on up and naked except for a pair of bright white Fruit of the Loom briefs. Percy managed to jump onto the Minotaur's back, pulled off one of the its horns and killed it by stabbing it in the side. Percy kept the horn as a prize, after Grover returned it from the scene of the battle.

The Minotaur almost kills Percy's mother, in fact, Percy is convinced she is dead, but she is actually held hostage by Hades and later released after Hades' Helm is returned.

The Last Olympian
In The Last Olympian, the Minotaur had reformed and is made a general for one of Kronos' Titan armies. He attacks Percy Jackson again when leading the Titan Army across the Williamsburg Bridge. This time it is harder to kill, as it is wearing armor, but Percy is more experienced now, and he has bathed in the river Styx to become invulnerable (except for the small on his back) so it was almost easy for him to send the Minotaur back to Tartarus where it belonged.