The X-Men are a very basic concept: some people are born with random genes that give them superpowers, often with weird side effects that make them, effectively, outsiders.
As it is, before Marvel decided that Inhumans are cool (they aren’t), the thing to do was to make everyone with superpowers and no clear origin a mutant and call it a day.
But whether or not longtime X-men character Longshot counts as a mutant is anybody’s guess.
Longshot started out as a solo act that eventually joined the X-Men. He’s wasn’t so much born as engineered. He came from an alternate reality called the Mojoverse, and was designed by a fellow called Arize the Creator for his master Mojo. What is Mojo’s deal? Well, he’s a large disgusting slug from a race of large, disgusting beings called the spineless ones. Mojo is basically in charge of that dimension, and what does he do? He keeps the masses entertained with gladiatorial combat crossed with reality television. Longshot, and that is his real name, was the star attraction before he escaped and fled to the regular Marvel universe.
As it is, Longshot isn’t quite a mutant, but he got to join the X-Men anyway. What powers does he possess? Well, he’s lucky. Really, really lucky. Probabilities work out in his favor all the time. Despite only having a total of eight fingers, he’s a lucky guy. For those times when he isn’t, he can heal very quickly. His skeleton is rather lightweight, making him very agile. He has good reaction time. He can sense psychic imprints on various objects and learn how the people who left them behind thing. And women (and gay men) find him incredibly attractive.
Fortunately, he doesn’t take advantage of that sort of thing too much. Longshot, when he settled down, was a one-woman man, and that woman was his X-Men teammate Dazzler, the disco-powered mutant pop star that could create light from sound.
Now, there were hints that another refugee from Mojo’s gladiator pit, Shatterstar, was in some way related to Longshot. In fact, an Easter Egg from an issue of X-Men went so far as to suggest Shatterstar was Longshot’s son.
The truth, when it came out, was even weirder. Both Longshot and Shatterstar found themselves teammates on the X-Factor team, and at one point, Shatterstar was blasted to the Mojoverse, where he met Arize the Creator. It seems throughout that world’s history, only one of the gladiators had not been a direct creation of Arize. That guy just appeared out of a flash of light in the sky one day, and Arize harvested his genetics to create Longshot. That made Shatterstar Longshot’s father and Longshot’s son.
That makes Shatterstar his own grandpa.
As for Longshot, his only goal was to bring freedom to all beings in the Mojoverse, a goal he may have eventually accomplished depending on what annuals you read.
I guess while he’s working on that, he may be too busy to help out anyone else. He may return some day. We should be so lucky.