Snake Mountain was not introduced until some way into the MOTU franchise. Previously, Skeletor's hideout had been described as "amongst the catacombs deep under Eternia", while the bible for the Filmation series listed his base as being Point Dread, located in the middle of the ocean.
The exact age of Snake Mountain is unclear, although in the season 1 episode "Golden Disks Of Knowledge" we learn that Skeletor built Snake Mountain with wisdom he learned from the Golden Disks Of Knowledge, which he stole from Zanthor, a member of the Council of the Wise.
The episode "Masks Of Power" reveals that many centuries ago, the Sword of the Ancients was thrust into a boulder of pure quartz, deep beneath Snake Mountain, and forgotten. According to the Sorceress, it is the most powerful weapon on Eternia, next to He-Man's Sword of Power.
The She-Ra: Princess of Power spin-off series reveals in the episode "Reunions" that during the invasion of Hordak & the Evil Horde, Snake Mountain was used as a Horde base. Eventually the Evil Horde was defeated by the Eternians. Skeletor, who at the time was a member of the Horde and a pupil of Hordak, betrayed Hordak, forcing the Horde leader to flee Eternia and abandon Snake Mountain.Skeletor would then take Snake Mountain and assemble a roster of evil warriors for his own evil purposes.
UK Masters of the Universe comic
In issue 22, The Secret Files of Scrollos attempted to explain why the Snake Mountain playset differed from its cartoon counterpart.
Snake Mountain was formed long ago, during the war against the Snake Men. The Council of Elders used their power to bind Serpos, mighty serpent god of the Snake Men, to a mountain on Eternia's Dark Hemisphere and turned it to stone.
During modern times, Skeletor took Snake Mountain for his own evil purposes.
The 1980s toy version of Snake Mountain looks little like its Filmation appearance. It is a purple mountain, with its most notable feature being a demon-like face on one side. There is a trap-door inside the gate, which is operated by a switch underneath. The toy also included a removable echo microphone, with the head of a snake near the top. The microphone stored inside the toy, with the head of the snake showing out the right side of the mountain. The demon face also had a lever located on the inside of the castle which allowed the user to operate the mouth of the face. This action combined with a user speaking into the echo microphone gave the appearance that Snake Mountain was talking in a frightening voice.
Snake Mountain made several appearances in the minicomics. Its depiction varies, but it tends to include both elements of the Filmation series as well as the appearance of the toy.
1987 Live Action Film
Although it did not appear in the film, Snake Mountain is mentioned at least once. Gwildor refers to Skeletor as the "Lord of Snake Mountain". This indicates that the location exists in the universe of the film.