Mangog, or "The Mangog" as it originally referred to itself, is one of the incredibly powerful Asgardian monsters who regularly oppose Thor. Twelve feet tall and demonic in appearance, Mangog has been portrayed as possessing incalculable strength, almost indestructible and capable of killing even Thor. Originally the sum total of the hatred of a billion billion beings that were once killed by Odin (comics), Mangog has terrorised Asgard on several occasions. Mangog appears to be dependent on tapping into psychic energies (such as hatred, fear, or feelings of devotion) felt by other sentient beings to sustain itself. Without such a source, Mangog shrinks to near nothingness.

Abilities
Mangog is pure destruction, and appears to be all but immune to physical force - despite Thor's best efforts. Mangog has usually only been stopped by cutting it off from the psychic energies that sustain it. In a brilliant counter-move, Thor once defeated Mangog by shoving Mjolnir into the beast's mouth and channelling a God Blast into it's very being.
Appearances
Thor #154 - 157, Volume 1.
Thor #195 - 198, Volume 1.
Thor #242; 244; 246 - 250, Volume 1.
Thor #20 - 25, Volume 2.