Mammoth Tanks are a hidden usable vehicle in the sequel to Command and Conquer, Tiberian Sun, but are usurped in hitting power by the Mammoth Mk. II. The Mammoth Tank has also appeared in the Red Alert series, called the Apocalypse in Red Alert II.
The Mammoth Tank has two giant cannons and surface to air missiles to defend itself with. If the tank existed in real life it'd be larger than any other current tank fielded, and too wide to be used in urban combat (it COULD fit on a highway, but not on the streets). Mammoth Tanks are also infamous for their ability to repair themselves back to half strength.
That guy defeated me with a Mammoth tank rush. I don't like dying, but, those Mammoths are SO much cooler than Zerglings. I'd rather be killed by a Mammoth than a Zergling.