Fantastic Four Replaced Invisible Woman With an INHUMAN
With a team as iconic as the Fantastic Four, one might assume the lineup remained consistently the same for decades. However, that's not exactly the case, as several characters have stood in for Susan Storm Richards aka the Invisible Woman. Two of these characters, in fact, were Inhumans - Medusa and Crystal.
This actually isn't entirely surprising, considering Medusa first appeared in Fantastic Four #36, and the Inhumans themselves appeared in Fantastic Four #45, with both the character and the augmented race of superpowered beings created by the legendary team of Stan Lee and Jack Kirby. Medusa, famous for her prehensile hair, would fight the Fantastic Four several times as a member of the supervillain team, the Frightful Four, having lost her memories of her Inhuman heritage. After regaining her memories, Medusa would take her rightful place with the Inhumans while maintaining a friendship with the Fantastic Four, paving the way for her to eventually join the team.
Related: Did X-Men's [SPOILER] Just Become King of the Inhumans?
During this time, Susan and Reed's toddler son Franklin was displaying massive powers. To prevent him from attracting the attention of villains like Annihilus, Reed rendered him comatose, prompting Susan to temporarily leave the team to care for her son. This lead Medusa to join the Fantastic Four, hoping to use her position to foster peace and understanding between humans and Inhumans. Medusa wouldn't be the only Inhuman to join the team, however - Crystal had previously joined the team while Susan Richards was pregnant with Franklin. She would remain with the team for quite a while, thanks in part to her mutual attraction to Johnny Storm, the Human Torch.
Ultimately, Susan would return to the team full-time, with Crystal and Medusa returning to their people. Ironically, much, much later, the Human Torch would join the Inhumans - as Medusa's lover and an ambassador. In any case, the Fantastic Four would reunite with its iconic lineup intact.
Much has been made of Marvel's ill-fated effort to emphasize the Inhumans over the X-Men when Fox still held the film rights to the mutant superheroes. There are numerous reasons why this didn't work - the two sets of characters are fundamentally different. However, Marvel's interest in the Inhumans was justified in one respect - they are an iconic part of the publisher's history. Medusa and Crystal's time on the Fantastic Four highlights the Inhumans' role in the Marvel universe, as well as their part in Lee and Kirby's creative legacy. Given their long history, perhaps characters like Medusa and Crystal will rejoin the Fantastic Four sometime in the future.