38. Sentinel

The X-Men have faced a lot of enemies who fear them simply for being different, but most of these foes have some sort of humanity that can be appealed to or reasoned with. Not so for the Sentinels. Giant, powerful robots, the Sentinels are perhaps the most frightening symbol of hatred against Mutants - machines created to hunt down those born different.

In their very first appearance, the Sentinels decided that the best way to protect humanity was to rule over it and that inclination was taken to its limit in the classic "Days of Future Past" story. There, we saw a future where the Sentinels are essentially the masters of humans and the scourge of Mutants, keeping them locked up in internment camps or simply murdering them outright. Who can forget the visual of a Sentinel disintegrating Wolverine himself?

Though they're mostly known for their uniformity and lack of individuality, there have been a couple of notable Sentinels, including their leader Master Mold and one particularly nasty version from the "Days of Future Past" timeline, Nimrod, who followed Rachel Summers back through time. Eventually, Master Mold and Nimrod would bizarrely be fused into another great X-Men threat, Bastian, whose seemingly human guise allowed him to spearhead Operation: Zero Tolerance, one of the most vicious government programs aimed against Mutants.

The X-Men version of the Terminators, the Sentinels are highly symbolic of the fear and hate Mutants must face.