The X-Men have been a powerhouse team in Marvel Comics since the 1960s. The roster has changed a lot over the years and has gotten so big at times there is no real definition to it, though that is about to change with the election of a dedicated X-Men team on Krakoa. There have been so many mutants on the X-Men it's easy to forget some of them.
From the original five X-Men to the thousands of mutants on the living island of Krakoa, the X-Men are a bit of a revolving door. Some have stayed around since the beginning. Others were here and gone in the blink of an eye.
10 Captain Marvel
Carol Danvers isn't really a mutant, but she is a mutated Kree-Human hybrid. She's also an honorary X-Man thanks to her long and complicated connection to Rogue. The future Captain Marvel spent years living inside Rogue's head after Rogue touched the Avenger in a battle.
They were uneasy companions, and often enemies, but that's all in the past now. Rogue acquired many of her powers from Carol, but well before Carol broke out a bunch of cosmic powers she now rarely uses.
9 Spider-Man
At least one version of Spider-Man is technically a mutant, but he never developed his powers because the bite of the radioactive spider more or less overrode his latent mutant genes. The Earth-13270 version of Spider-Man eventually manifested his mutant powers when his spider-powers wore off.
The Earth-616 version was very briefly a member of the team. Peter Parker took on the role of a special guidance counselor at the Jean Grey School of Higher Learning back in 2014 when he suspected one of the students of being an imposter.
8 Maggot
There have been a lot of X-Men line-ups over the years. In the 90s, the ranks started swelling dramatically and a lot of characters were introduced. Some were good. Some were not. Maggot was a pretty significant member of the team during this era but has mostly been forgotten.
He didn't inspire a lot of interest or affection with his weird powers, which included having his digestive system exist in the form of two alien slugs that wrapped around his body.
7 ForgetMeNot
No one can fault ForgetMeNot for being forgotten. That's essentially his power. His mutant ability is to make people forget him immediately and so no one ever really knew that he was a member of the X-Men. This went so far that Professor X had to set a reminder to remember ForgetMeNot.
He was later a member of one of X-Force's most powerful rosters, though his life ended suddenly and despairingly when Fantomex shot him. He was utterly forgotten again and would have been forever, if not for the efforts of Hope Summers.
6 Cloak And Dagger
Cloak and Dagger are a pair of mutants who only discovered the true nature of their power after their interaction with The New Mutants. Before, Tyrone and Tandy, Cloak and Dagger respectively, had been associated with Spider-Man and other heroes.
They eventually joined up with the larger mutant network by becoming members of the Dark X-Men. The two teen heroes were the focus of their own live-action series on Freeform, but it has since been canceled.
5 Sage
Sage's history with the X-Men goes all the way back to 1980 when she first appeared in X-Men #132. She only appeared as the superhero Sage years later, in issue #109 of the second volume of the series. Sage joined the X-Men on account of Storm and used her broad array of talents to help the X-Men out of a lot of jams.
She used her talent to manifest latent mutant abilities combined with her powerful intellect to heal Beast after a traumatizing battle. Though she had a long history with the team, she's since been largely forgotten.
4 Lifeguard
Lifeguard is a member of the X-Treme X-Men from the early 2000s. Heather Cameron was a lifeguard in Australia when she and her brother, Davis, were saved by Storm and Thunderbird. Inspired by their generosity, Lifeguard joined up.
Her status as an X-Man and a mutant didn't last long though. It was revealed that was actually a member of the Shi'ar race. She remained part of the team after this but then simply disappeared, completely forgotten by the creative teams, until she recently made a cameo on Krakoa.
3 Slipstream
Her brother Davis Cameron also joined the X-Men when he discovered both he and his sister were mutants. Davis had been a surfer before joining and carried his love of the waves into his mutant ability. He could create a warp wave that teleported people across great distances.
Davis became one of Storm's most unlikely love interests during his tenure on the team. That didn't lead anywhere though and he was depowered on M-Day. He's since been MIA.
2 Firestar
Firestar is likely best known for her stint with the New Warriors, but she was part of the X-Men as well. Angelica Jones got a job as a physics teacher at the Jean Grey School of Higher Learning around the same time as Spider-Man and also joined the X-Men.
Most people probably don't remember since it was so brief. She's one of the few characters in Marvel to be part of both the X-Men and Avengers and famously made her debut in animation as part of Spider-Man And His Amazing Friends.
1 Thunderbird (Neal Sharra)
Longtime X-Men readers will of course remember Thunderbird, but some may not remember Thunderbird III. Neal Sharra joined in the year 2000 and has no connection to the original Thunderbird outside of the name. He could convert solar energy into kinetic power and turn his entire body into solar plasma.
He was part of the X-Treme X-Men and was one of the few members to keep his powers after M-Day. He later was simply forgotten, along with Lifeguard, his romantic partner, as they searched for Davis Cameron.