Tirek is a power-hungry Centaur from Equus, who originally arrived into Equestria to take over, only to be imprisoned in Tartarus. He is also one of the antagonists of the Season 1 finale.
Physical Appearance
Tirek is a large, muscular Centaur, with a pair of large horns on the side of his head. He has the lower half of a cloven-hooved horse with dark grey fur, silver hooves and a silver mane and tail, where as the upper half of his body is more humanoid with black fur and red skin with a monkey-like face with a thick breard. He also wears a sort of metal ring around his chest, and a ring in his nostrils like that of a bull or cow.
Personality
On his first appearance, Tirek is shown as a clever creature with a silver tongue, good enought o be able to manipulate Discord into his good graces by convincing him that he'll give him freedom over friendship, something he considered better. As he grew stronger though, he began to drop that act for a more brutal nature.
History
Tirek had arrived to Equestria once before, with a similar goal to his current one to take over the land. However, Celestia and Luna had found him and he ended up imprisoned in Tartarus.
Tirek eventually escaped again however, and began to drain power from other ponies, when he eventually ran into Discord. He honored him for his own escape, him seemingly aware of his imprisonment as well, but only finding out he was there to imprison him as well. Tirek convinces him though that he will offer him Freedom if he agreed to help him with his own goal, wanting to see Equestria's world turned "upside-down". After making the deal, he went on a rampage and draining all the magic he could locate, including the magic of the Alicorns, and later on the Gems. He ended up in a brawl between them, and only being stopped when a gemstone on him becomes cracked by Lion, and as such lost all of his abilities he had gained from it. He was then placed back into Tartarus alongside Sombra.
Abilities
Power drainage
His signature ability is to drain other around them of their abilities and strength, and able to use those same effects as his own. This is notable when he uses Gem weapons after draining the powers from the Crystal Gems themselves.