Monster Mountain is the mountain used in the Monster in My Pocket video game, Monster in My Pocket comic books, and The Big Scream animated special. In the video game, Monster Mountain is Warlock's headquarters, which houses the evil monsters. In the comic books, Vampire and Warlock, along with the monsters they've allied themselves with, hold a meeting at the mountain. Warlock's scheme shrinks all of the monsters on the mountain down to a few inches, forcing them to get lost. The Big Scream mentions the mountain briefly as a prison, which vampire , Medusa, and Swamp Beast are captives in. However, vampire's plan shrinks them here, also, and they get swept away to California.
There was a playset which held all 48 Series 1 monsters called the Monster Mountain, which would hang on the wall and display the figures. In the 2006 series, there was a playset called the Haunted Mountain, which came with Poltergeist and Vampiress, and was very similar to the original Monster Mountain. However, it was more of a playset than a display case.