The Doom Patrol, which debuted in comics three months before everybody's favorite, more marketable mutants.
Unlike the X-Men, the Doom Patrollers were once normal people who suffered an accident that disfigured them but also gave them superpowers. Shunned by the world for just being plain ugly, the freaks were gathered by Doctor Caulder, a paraplegic, who thought that maybe the world wouldn't dislike them so much if they used their powers to save the normal people's asses from giant robots once in a while.
If this sounds somewhat familiar to you, it's because the same thing as X-Men with the only difference that the smart guy in the wheelchair was bald in one and X-Men uses mutants as an allegory for minorities instead of people with elephantiasis or whatever the heck Doom Patrol was going for.
Even the tag line is the same! At least make an effort, guys!
Possibly, the most unnecessary thing borrowed by X-Men was the name of the Doom Patrol's enemies: The Brotherhood of Evil. In Doom Patrol the name made sense; because they were a group of evil assholes, which got together to do a*****e things. There was never any confusion about what the group was about.
On the other hand Magneto stole the name, added the word mutant at the end of it and then whined endlessly about how humans persecuted and hated him. Maybe people hated you, Magneto, because your group's name was The Brotherhood of Evil Mutants and you went around the world trying to wipe out humanity?
How successful would the American Paraplegia Society be if they called themselves the Brotherhood of Child Molesting Guys on Wheelchairs? Magneto's weak PR skills aren't the only reason the original Brotherhood looks awesome by comparison ...
A brain in a case and an armed gorilla? How is Magneto more famous?