I don't know about his existance outside of where I know him from, but Thanos is to me as Frieza is to Goku. Old news. I first met Thanos YEARS ago when I first played "Marvel Super Heros" the arcade fighter. Oh he was hard then, but like Goku and his fights, the levels increased as time went on, leaving past battles in the dust cuz of all the games that spun off after it! X-Men vs. Streetfighter, Marvel vs. Capcom (fav).
LOL. Seriously though, I always thought without those gems he was kind of like Superman's Darkseid. I mean, you sometimes see Marvel characters mirror DC characters or vice versa.
At least, I heard he was. I know he greeted the Marvel Captain Marvel (aka Mar-Vell. There are way too many Captains Marvel runnin' loose.) when he was killed off. I don't know what his powers were, except whatever the writer wanted him to have. His deal was that he was trying to marry a personified female Death, and to do this he had to destroy half the universe, or something. I dunno. Space comics are relentlessly dull.
And he was a lot tougher in that game than the more recent bosses. I beat him ONCE, with Juggernaut. I routinely beat Apocalypse, Abyss, and Cyber-Akuma by switching fighters. (Onslaught was one cheap-ass bastard, though.) You can play Thanos in the MvC2 game, and he's okay, but I prefer the Cable-Juggie-Colossus combination.