Once a member of the Swamp Men of the Savage Land, a tropical jungle hidden away in the frozen wastes of Antarctica, Amphibius was one of several of his people who were artificially mutated through technological means by the self-styled mutant master of magnetism, Magneto, so as to grant them superhuman powers. On Magneto's behalf, the Mutates subsequently battled Ka-Zar, the Lord of the Savage Land, and his allies in the team of superhuman mutant adventurers known as the X-Men. After the destruction of Magneto's machines by the X-Man Marvel Girl, the Mutates returned to their original non-superhuman states.
However, the Mutates later regained their powers thanks to the effects of the technology that changed them, causing them to regain their superhuman abilities should they ever lose them. Renaming themselves the Beast Brood, the Mutates proceeded to capture scientists in the hope of forcing them to mutate other Savage Land natives into superhuman beings. The costumed adventurers known as the Avengers and the Norse goddess Sif came searching for the scientists and defeated the Beast Brood, delivering them to legal authorities in Chile, the nearest nation to the Savage Land.
The Mutates eventually escaped custody in Chile and returned home to the Savage Land. One of the Mutates, Brainchild, assumed leadership of the group. Under Brainchild's leadership, the Mutates clashed with the winged X-Man named Angel, the costumed crimefighter Spider-Man, and Ka-Zar. The trio of heroes were searching for Karl Lykos, the human alter ego of the mutant pterosaur known as Sauron. Captured by the Mutates, the heroes were freed by Lykos but at a terrible cost, as Lykos became Sauron once more.
The Mutates subsequently came under the leadership of Zaladane, a Savage Land priestess who once served the being known as Garokk, the Petrified Man. Zaladane subsequently led the Mutates in raids on Antarctic research stations, plundering Chilean cities, and kidnapping scientists in an effort to cement her seat of power in the Savage Land. Zaladane then dispatched the mutates to kidnap the former X-Man named Polaris, who was unknowingly Zaladane's sister. Zaladane stole her mutant ability to manipulate magnetism and the Mutates clashed with the X-Men who had come to rescue their missing friend.
Following Zaladane's death at the hands of Magneto, the Mutates came under the leadership of Sauron and clashed once more with the X-Men who were searching the Savage Land for clues as to Magneto's whereabouts.
Brainchild subsequently regained his position as leader of the Mutates. When the X-Men were helping a band of Saurids return to the Savage Land, Brainchild seized the opportunity to settle an old score with the weather-manipulating X-Man named Storm. Brainwashing her, Brainchild sent her and the Mutates into battle against the rest of the X-Men, but they were defeated. The X-Man Sage subsequently reversed Brainchild's mental manipulation of Storm, and the Mutates were turned over to the leaders of both the Saurids and the Savage Land tribes, who seek for all denizens of the jungle to coexist in peace.
Amphibius was originally a native of the Savage Land but was transformed by Magneto into one of his Mutates. When Magneto disappeared, Amphibius and the other Mutates found a new master, Mister Sinister. After the defeat of Barbarus and Vertigo, Sinister dispatched Amphibius to capture Magneto and Professor X who had become stranded in the Savage Land. He was successful until the company was attacked by an enraged elasmosaur, where Magneto and Professor X escaped.