Marvel's first family has been around since 1961, and while their core members are pretty established, from time to time, they have accepted some help from their friends. Mr. Fantastic, the Invisible Woman, the Human Torch, and The Thing comprise the heart and soul of the famous super-team, but even they need sabbaticals every now and then. During those times, other Marvel heroes have been called on to don the circled 4 on their chest and lend a hand with the disaster of the day. Let's take a look at some of those who've helped to round out the team over the years.
Crystallia Amaquelin, member of the Inhuman royal family and former girlfriend of the Human Torch, filled in for Sue back in 1968 during her pregnancy with Franklin. Crystal is quite a powerful character in her own right, possessing the ability to manipulate the elements - fire, earth, wind, and water - allowing her to do largescale things like creating tornados and earthquakes, or something as simple (in comparison) as using the wind to carry her, not unlike Storm of the X-Men. Crystal would eventually go on to join the Avengers as well. Bonus, her fellow Inhuman and sister Medusa also spent time with the FF.
9 Power Man
Well before Netflix and Mike Colter made Luke Cage a household name, Power Man was relegated to the pages of Marvel, where he first appeared in 1972. By 1978, he had teamed up with Danny Rand to form Heroes for Hire, and even worked briefly for the Fantastic Four, hired by Reed Richards to fill in for Ben Grimm who had recently lost his powers and was no longer The Thing.
Much like Crystal, Cage too would eventually go on to become a member of the Avengers as well.
Following the events of the Secret Wars back in '84, Jennifer Walters, better known as She-Hulk, reappeared on Earth with Reed and Johnny, while Ben elected to stay behind on Battleworld for a time. While Ben is gone, She-Hulk acts as the team's muscle and actually hangs around for quite a while, sticking around for a couple of years until The Thing finally returned in November of 1986. At this point She-Hulk left the FF and returned to her previous gig as - you guessed it, an Avenger.
7 Mr. Fixit/Hulk
A few years after She-Hulk's extended run with the team, cousin Bruce got a shot as well, sort of. Part of the "New Fantastic Four," Hulk's Mr. Fixit persona joined an all-star team, recruited to avenge the original members of the FF after their supposed slaying. For three issues in 1990/91, and again for a couple more in 1993, Mr. Fixit (and later Hulk) played the part of the strong man for the team.
6 Ghost Rider
Another member of that "New Fantastic Four" team was the '90s newest Ghost Rider, Danny Ketch. Looking similar to his predecessor Johnny Blaze, Ketch's Rider was a little more updated for the era and in the midst of a real popularity boom.
The team gets duped due to some Skrull trickery, and thinking the original FF is dead they end up in a fight with Mole Man and are forced to save a brainwashed Reed Richards.
Rounding out the "New Fantastic Four" were Spider-Man and Wolverine. Spider-Man has been tied to the FF plenty of times, attempting to join the team in his early days, and eventually becoming part of their Future Foundation, but Wolverine's involvement is more limited. Initially tricked into working with the others by the skrulls, and later recruited by Dr. Strange to arrest the Human Torch, the most interesting instance of this team's presence for a Wolverine fan is likely when we see them in the Age of Apocalypse.
Back in the mid-nineties, in need of another big-brain on-site following the supposed death of Reed Richards, Scott Lang is hired on at the Baxter Building as an electronics expert.
It doesn't take long before danger strikes and Scott reveals that he is, in fact, Ant-Man. Lang sticks around for the next 30 or so issues before Reed eventually returns. Scott goes on to later join a new Future Foundation team, along with some other ex-FF members (including She-Hulk) as temporary fill-ins for the original team.
Wolverine isn't the only member of the X-Men's extensive roster of supers to have spent some time with the Fantastic Four. Following the events of Marvel's Civil War, Reed and Sue decide they need to step away for a bit and work on their marriage, and their roles are taken up by Storm and her husband, Black Panther. Join Johnny and Ben, Storm and her husband is able to offer a new dynamic to the team, and waste very little before venturing into the cosmos and stirring it up with some Gods.
2 Black Panther
T'Challa has a long and storied history with the Fantastic Four and has been making appearances in their book since 1966, so it seemed fitting that he and his wife (Ororo Munroe) were given the task filling in here after the events of Civil War left the superhero community in shambles and the Wakandan Embassy in rubble. They weren't on the team for too long, but Storm and T'Challa did live in the Baxter Building for a time, using it as the new Embassy and racking up encounters with Silver Surfer, Deathlock, The Watcher and more.
1 Devil Dinosaur
Devil Dinosaur first showed up in Marvel comics in 1978, but despite impressively squaring off with Godzilla a few times, wasn't a consistent presence in the continuity. Ben Grimm encounters a fake Devil during his solo series in 1986, but the real deal doesn't show up with ties to the FF until much later. Though short-lived, the dinosaur from Earth-78411, along with his companion Moon Girl, did join up with Ben and Johnny for a time last year in the pages of Moon Girl and Devil Dinosaur.