The Mountain Giant is one of the many varieties of Giant native to the various worlds of Dungeons & Dragons. A close relative of the more common Hill Giant, Mountain Giants inhabit the taller peaks - information beyond that differs greatly depending on edition.
In Mystara, Mountain Giants are described as physically reminiscent of both Hill Giants and Stone Giants, and culturally are somewhere in between as well. Largely reclusive, they are not above hiring out as mercenaries for profit, and in fact often strike up symbiotic relationships with dwarves; the dwarves furnish them with arms and armor, and the mountain giants squish anybody who tries to kick the dwarves around.
In Advanced Dungeons & Dragons, Mountain Giants are characterized as being smaller than Hill Giants (14ft tall and 2,000 pounds to the hill giant's 16ft tall and 4,500 pounds), but notably smarter. They are characterized as a quiet and reclusive race, slowly dwindling into non-existence because only 1 in four babies are female; this has led to the race practicing polyandry, but that can't stave off the slow decline in the population as fewer and fewer fertile females are born to replace the dead. Mountain giants are isolationists, who harbor no true malice towards other races and just want to be left alone, eking out a simple lifestyle of hunting and gathering.
In Dungeons & Dragons 3rd Edition, on the other hand, Mountain Giants are enormous; at over 40 feet tall, they're one of the biggest of the giant races. They've also devolved to become more savage and vicious; like their hill giant cousins, these mountain giants are cruel, capricious brutes who delight in slaughtering anything smaller and weaker than themselves for fun. They are no longer presented as a dying race, either.
Mountain Giants debuted in AD&D 1e's Fiend Folio and BECMI's Master DM's Book. They were updated to AD&D 2e first in the Monstrous Compendium Forgotten Realms Appendix, being reprinted from here into the Monstrous Manual. They made their 3e debut in the Monster Manual 2, and haven't reappeared since.