Centaurs look like horses with the upper-portion of a man's torso where their head would normally be. They were generally considered to be uncivilized, and that was why they were part beast. All centaurs were male. They came into existance when a mortal named Ixion coupled with a cloud that he thought was the goddess Hera.
Cheiron--"The Great Teacher". He was more humane and civilized than most men, and taught many of the great heroes including Herakles, who accidentally shot him with an arrow poisoned with the venom of the hydra. Cheiron was in immence pain, but was immortal and couldn't die, and hydra venom had no cure. He gave his immortality to Prometheus so that he could die and escape the agony of his wound. After his death he was put into the sky as the constellation Sagittarius.
Nessus--He tricked Herakles into letting him carry Deianaria accross a river, and planed to abduct her before Herakles could cross. Herakles found out, and shot Nessus with an arrow. Before Nessus died he told Deianaria that when applied to a cloak, his blood would make the wearer of the cloak love her forever. She believed him. She put it onto one of Herakles' cloaks, but instead of making him love her, it burned his skin, and to escape the pain he was forced to kill himself. Deianaria hung herself after finding out what she had done.
Pholus--While chasing a boar, Herakles stoped at his house and ate dinner there. All was fine and dandy until Pholus hesitated to serve the Centaurs' common wine with the great hero. Herakles insisted, and Pholus served the wine. The other centaurs smelled the wine, and fought Herakles for it. He killed or chased away those who attacked him. Pholus died in the battle, having droped one of Herakles' poisoned arrows on his own foot.
Eurytion--He tried to violate king Dexamenos' daughter, but Herakles rescued the girl and killed Eurytion.
Elatos--The Centaur that Cheiron was protecting when Herakles shot Cheiron.
Abas--A skilled hunter.