Peter Maximoff

Quicksilver is a mutant.

History
In X-Men Origins: Wolverine, he makes a brief cameo as a prisoner of William Stryker in his mutant testing facility on Three-Mile Island. He is tied down via bungee cords to prevent him from building up enough kinetic energy to escape. He, along with the rest of the incarcerated mutants, escape with the help of Professor X.

Trivia

 * In X2, the name "Maximoff" appears on a computer screen in a list of mutant names.