Though they were founded as a team of mutants, the X-Men have welcomed a whole host of heroes over the years, even those who aren't mutants at all. While its most famous members might have been born with their unique gifts, some came upon them in other ways.
From villains turned heroes such as Juggernaut to beloved speedsters such as Quicksilver, plenty of non-mutants have been part of the squad. While many have joined, only the best left an impression on readers and are synonymous with the mutant team even though they themselves weren't born as mutants.
Moira MacTaggert
Unlike a lot of X-Men members who gained powers through other means, Dr. Moira MacTaggert had no extraordinary abilities whatsoever. As a brilliant geneticist and sometimes love interest to Professor X, Moira had no abilities but was an important member for many years.
Her scientific knowledge often came in handy on missions, and she was usually the brains of the operation, alongside Charles Xavier. Moira's commitment to the mutant cause was so strong in fact, that she was even the only human character to contract the dangerous Legacy Virus because of her close association.
Puck
Though Puck is most known as one of the best members of Alpha Flight, the pint-sized hero was briefly an X-Men member as well. Born in Canada, Eugene Judd was a towering man who sought a mystical blade. After taking the sword, he was caused to shrink by a spirit that possessed the sword, and he was granted near invulnerability as well.
Despite not being a mutant, he made a great addition to X-Force, and fit right in amongst his mutant brethren. Puck's abilities rivaled that of powerful mutants, and the chip on his shoulder emboldened him to be a ferocious fighter as well.
Mimic
Calvin Rankin, aka Mimic, wasn't an original member of the X-Men but he joined shortly after they were founded. An on-again off-again hero, Mimic has the ability to copy the powers of others, and usually takes on certain physical characteristics of whoever he's mimicking.
Created when one of his father's lab experiments went wrong, Calvin's powers were never of a mutant nature. Nevertheless, after briefly sparring with the X-Men, he did join their team for a brief time and even achieved a rank within the team hierarchy. Unfortunately though, Mimic's quest to make his powers permanent, has seen him turn bad more often than good.
Fantomex
The Weapon Plus experiments created many non-mutant superhumans, and Fantomex was one of the strongest. Evolved by a series of genetic enhancements, Fantomex's life story remains a mystery, but his powers of invulnerability and superior intellect is evident.
After escaping his fate inside the program, Fantomex found a safe haven within X-Force, and quickly became an important asset to the X-Men's darker counterpart. Charging headlong into conflicts with both guns blazing, Fantomex is one of X-Force's most powerful members, and an extremely skilled thief as well. Darker in tone than most X-Men members, Fantomex crosses the line into anti-hero territory quite often.
Omega Sentinel
Technology is often at the heart of created superhumans, and Omega Sentinel started out as an instrument of destruction. Born as Karima Shapandar, Omega Sentinel was specifically created to seek out latent mutants and destroy them as soon as their powers manifested.
Imbued with cybernetic technology, Omega Sentinel was at the whim of Bastion, but eventually was saved by Charles Xavier and Magneto. Her cybernetic hardware has made her susceptible to many possessions, and Omega Sentinel hasn't always been a force for good, even if it is unintentional. In one of her better moments, she attended Jean Grey's school, and was a member of the X-Men for a brief time.
Lockheed
Most X-Men members have a humanoid appearance, even if they are technically not human at all. Lockheed on the other hand, came to the team as a non-mutant, and a non-human as well. Discovered when the X-Men were kidnapped by The Brood, Lockheed is a small dragon-like alien that became a longtime companion of Kitty Pryde.
On top of being extra durable, Lockheed can shoot fire from his mouth and fly like a typical dragon. He was treated like a house pet before eventually earning a place as a legitimate member of the team, and actually saved them on a few occasions.
Warlock
Warlock's origins weren't of the earth, but his backstory was somewhat similar to his mutant brethren. Born of a mysterious alien race that merged technology and organic life, Warlock became an outcast from his society after breaking with their tradition of patricide. He fled persecution and ended up on earth with the X-Men.
Warlock's techno-organic body is capable of constantly changing its shape, and he can instantaneously adapt and self repair whenever needed. He soon found himself a part of the New Mutants team, and he found a community of likeminded outcasts to enjoy. Though his powers were great, it was his passive nature that made him a mutant within his own species.
Cloak And Dagger
Though they made their debut in the Spider-Man comics, the team of Cloak and Dagger also were integral parts of the X-Men canon. Originally teens from very different backgrounds, the pair gained their powers when they were forcibly injected with a synthetic drug.
While they mostly stuck to their own personal adventures as a duo, the team was courted by Norman Osborn and soon joined his Dark X-Men squadron. In terms of powers, Dagger is the embodiment of light, and can harness its energy, while Cloak can channel the dark forces of another dimension. Debate raged about whether or not the team were mutants or not, however in recent years it has been revealed that their DNA isn't mutated.
Quicksilver
Pietro Maximov and his sister Wanda were always believed to be mutants, but in recent years their backstory has been retconned. Gifted with extraordinary speed by the High Evolutionary, Pietro took the moniker of Quicksilver and has been a staple of both the X-Men and The Avengers ever since.
Being one of the first 10 members of the X-Men, Quicksilver has been around long enough to be involved in some of Marvel Comics biggest storylines. Because he was believed to be a mutant for so long, he fit right in with his mutant family, and suffered from the prejudices against them. His talents made him an incredible asset to any team that he was a part of, and he has returned frequently to help out his original squad.
Juggernaut
Though he is usually characterized as a ruthless villain, Cain Marko's alter-ego Juggernaut has actually fought alongside the X-Men on several occasions. The towering behemoth of pure strength gained his powers when he found a magical gem in the Temple of Cyttorak. The gem made him Cyttorak's avatar on earth, and thus his reign of terror began.
The step-brother of Charles Xavier, Marko harbored a deep resentment against his sibling because he always felt inferior. Eventually though, the two reconciled and Juggernaut fought with his longtime foes as a member of the X-Men, and then with Excalibur. His place among non-mutant X-Men is a lofty one because of the scope of his immense power. As a villain he was nearly unstoppable, and as a hero he could channel that energy into productive uses as well.