7) Mastermind
The X-Men universe is so overpopulated that the movies regularly shove comic characters into the narrative wherever they can put them (which often causes continuity issues when they reappear down the line). One prominent character hidden in the movies is Mastermind, a powerful mutant foe of the team.
In X-Men 2, General Stryker callously uses his lobotomized son Jason to manipulate Professor X into wiping out all mutant kind. Going by his mind control abilities and first name, it can be gleaned that he is a very loose adaptation of Jason Wyngarde, known as Mastermind in the comics. He’s usually a member of the Hellfire Club, but they later appeared without him in X-Men: First Class.
Though he is one of the bigger X-Men comics villains, this version of Mastermind is so far removed from the familiar character that it’s easy to forget that he made an appearance in the movies at all.