Mastodon was revealed to have been a member of Team X , and later a test subject of Weapon X. He was a well-built man who was well-trained in the use of firearms.
He appeared in the flesh for the first time as an old man, although he was supposed to be aging much slower than other humans due to an aging-suppression factor given to him at Weapon X. As the aging-suppression had somehow failed, he ultimately died of old age.