In the movies, Magneto is Erik Lehnsherr. Maximoff is the last name given to Quicksilver, from her mother. Quicksilver never knew his father, so he never got Magneto's last name. However, in the comics, Erik Lehnsherr is a fake name, and Magneto's original name is actually Max Eisenhardt, and he only moved to the name Erik Lehnsherr later in life. Between that, he changed his name to Magnus as well.