The villainous unicorn who serves as the main antagonist in the 2017 My Little Pony film is Tempest Shadow.
Tempest is the Storm King’s commander and leads the invasion of Equestria. She captures the princesses and pursues Twilight Sparkle because the Storm King promises to repair her broken horn. Her real name is Fizzlepop Berrytwist.
Tempest’s horn was damaged during an Ursa Minor attack when she was a filly. Afterward, her unstable magic and rejection by former friends left her embittered. She believes the Storm King can restore what she lost, but he ultimately betrays her after using her to obtain magical power.
A common mix-up is calling the Storm King the unicorn villain. He is the film’s satyr-like conqueror and supreme antagonist, while Tempest is the unicorn lieutenant who drives much of the story’s conflict. Twilight’s compassion eventually helps Tempest reject him and begin her redemption.