His name comes from the fact that he is very large, comes from high up in the mountain country, and fucks mammoths.
It wasn't until years later after the Penultimate Exigency when the Revengerists were desperate to re-fill their roster with any ole Awesomes around (due to the horrific deaths and time-erasures of their previous comrades), that Odd Jog remembered his friend would be perfect for those special operation missions where... a mammoth needed fucking.
He didn't last too long, although highly popular with the sick degenerates who read Revengerist comics, because on one dimensional-jumping adventure in 2007 the team happened upon an unspoiled alternate Earth full of luscious mammoths, and the Mammoth Mountain Fucker was tempted to remain there forever. Even though there were no mountains. He just loved mammoth fucking that much.
- "Sorry guys, I just tripped and my dick fell into this mammoth."
Interestingly, the fact of the matter is the Mammoth Mountain Fucker has never even met the beautiful and seductive Mammoth Princess of Space, as they were not affiliated with the team during the same timestreams.