17. The Sentinels
It's not easy being a mutant in the Marvel world. They're treated like sub-humans, and from time to time maniacal world leaders even try to kill them off completely. That's where the terrifying Sentinels come in, giant mutant murdering robots that are basically created to bring about a mass mutant genocide. Fortunately, the Sentinels never achieved what they were programmed for, but remain one of the many looming threats in the X-Men world.