Tirek is a Generation 4 Antagonist, that appeared in the show My Little Pony: Friendship is Magic. He is a centaur-like being and was directly inspired by the Generation 1 character.
Tirek is the main antagonist of Twilight's Kingdom: Part 1 and Twilight's Kingdom: Part 2, the season 4 finale of My Little Pony: Friendship is Magic. This version of Tirek has the ability to absorb the magical abilities of ponies, and use these stolen abilities to enhance his own power. Victims of his power are visibly weakened, lose their cutie marks and gain dull gray eyes.
In the distant past, Tirek and his brother Scorpan invaded Equestria and sought to steal the magic from all the pony races. However, Scorpan would befriend the ponies and turn against his brother by alerting Princess Celestia and Princess Luna. The Princesses would imprison Tirek in Tartarus, while Scorpan would return to his own land.
Tirek would escape from his prison when Cerberus escaped from his post in "It's About Time". He would remain weakened, but began stealing Unicorn magic to strengthen himself. Celestia would send Discord to capture Tirek, but Tirek managed to convince Discord to turn against the ponies. After gaining more strength, Tirek was able to remove the flight abilities of Pegasi, and the strength of the Earth Ponies. Realizing that Tirek would seek Alicorn Magic, Celestia, Luna, and Princess Cadance, would transfer their power to Twilight Sparkle; they believed that Tirek would be unaware of Twilight's new status as an Alicorn Princess, allowing their magic to remain hidden. The plan succeeded initially, but Discord would make Tirek aware of Twilight. After capturing and depowering Applejack, Rarity, Fluttershy, Pinkie Pie, Rainbow Dash, and Spike to lure Twilight, Tirek took Discord's magic as well.
The battle between Tirek and Twilight Sparkle began with him destroying the Golden Oak Library. The battle was intense, but quickly became a stalemate, so he offered a trade: her friends (including Discord) for the magic she currently possessed. She reluctantly agreed. After Twilight got Scorpan's medallion from Discord, which became the final key, the mane six received the power of the Tree of Harmony and defeated Tirek, sending him back to Tartarus and reversing the damage he had done.