Mystic Mountain is an amusement park that Robbie and Rex went to. It is only mentioned by Robbie in the episode Rex Dies. Robbie was too scared to go on one of the roller coasters, and Rex calls him a weenie for it. Robbie rode on it anyway and loved it, even though he peed in his pants on the loop-to-loop. One ride there is called The Demonator (which was actually seen in Drake & Josh and Zoey 101), possibly the one Robbie rode.
In other shows
Mystic Mountain was seen in the Drake & Josh episode "The Demonator" and the Zoey 101 episode "Roller Coaster." The park is a parody of Six Flags Magic Mountain in Valencia, California, and all scenes in a Dan Schneider show taking place in Mystic Mountain are actually filmed at Six Flags Magic Mountain.