Yetrigar was an ancient creature who was mutated by radiation.
Battle against Godzilla and Red Ronin
An underground nuclear test in Arizona conducted by the US Department of Defense had unforeseen side-effects. The radiation from the test was channeled into an underground rift, a subterranean passage stretching from Arizona to Alberta, Canada. In Alberta, the radiation came to fill an ice cave, within which a hairy, near-human creature was encased in a glacial tomb, one of the legendary Sasquatch. The Sasquatch was freed from the ice by the radioactivity and revived. The newly revived monster was pained by the continuing effects of the radiation on his body. His mind filled with rage, the bigfoot turned south, towards where he instinctively sensed the source of his affliction originated. As he strode southward, however, the radiation began to make the man-monster start to grow.
The Sasquatch monster encountered SHIELD's Godzilla Squad as they were tracking Godzilla in the Arizona desert. Hurling a huge boulder at the SHIELD helicarrier Behemoth, Yetrigar put SHIELD out of the hunt, and began to himself track their monstrous quarry. Yetrigar ambushed Godzilla at the rim of the Grand Canyon, causing both monsters to topple into the natural wonder. The battle soon escalated when Red Ronin arrived to try and intervene and prevent the monsters from killing each other. However, even Red Ronin could not control two such awesome creatures and was at last forced to fire on the wall of the canyon, causing a rockslide which buried Yetrigar, seemingly killing the giant bigfoot.
Battle against the West Coast Avengers
Years later, an explosion in the canyon released Yetrigar, who instantly attacked the superhero team the West Coast Avengers, who were vacationing at the Grand Canyon at the time. He was subdued after a long battle by the size-changing Dr. Hank Pym (aka Yellowjacket), who managed to render the brute unconscious by enlarging a portion of Yetrigar's inner ear. Dr. Pym would use his shrinking gas to reduce Yetrigar down to a manageable size so that the giant would not prove quite so much of a public menace.