This is another of my explorations into fear. On writing these, a factor besides the unknown has emerged. Namely, how close are we to the unknown? For example, mutants are normal people who have been genetically twisted. Whether through inbreeding or experimentation, mutants are almost us. Recently, I wrote about The Hills Have Eyes and their mutant clan. They all are human but each has some characteristic (or three) that sets them apart.
In comic books, there are evil mutants who believe mankind's time is over and their time has begun (see folks like Magneto). For horror mutants, usually the motivation of a mutant is to remain separated from the rest of humanity and, if someone crosses into their domain, bad things happen. I think about movies like CHUD and Nightbreed, where mutants dwell in the sewers. The aforementioned Hills Have Eyes where the mutants live in the desert and even movies like Spring where the mutants can live concealed among us. In horror, they make themselves the other. In movies like the Goonies, mutants like Sloth can be accepted into the ranks of normal humans.
Mutants also get a bit of a boost as they are something you can encounter in the real world. Genetic mutations happen all the time. The arguably best episode of X-Files deals with a group of inbred killers. Likewise with Texas Chainsaw Massacre. So I pose a question to you now: what is scarier, stumbling into a hostile mutant's den or living next door to a sociopathic serial killer?