The centaur-like villain who steals magic in Season 4 is Lord Tirek.
Tirek is the main antagonist of the two-part fourth-season finale, “Twilight’s Kingdom.” After escaping Tartarus, he drains magic from ponies across Equestria, growing larger and more powerful with each victim. He eventually targets Discord and Twilight Sparkle, seeking enough power to conquer the realm.
Tirek’s magic-stealing role distinguishes him from the other listed villains. Cozy Glow becomes a major antagonist later, King Sombra rules the Crystal Empire, and Discord begins as a chaos-wielding villain before becoming an ally. Tirek is also a newer version of Tirac, the centaur-like villain from the 1984 special Rescue at Midnight Castle.
A common mix-up is the spelling: the original character was named Tirac, while Friendship Is Magic uses Tirek. In the Season 4 finale, Twilight and her friends defeat him with the Rainbow Power and return him to imprisonment in Tartarus.