Trevor 2: The Search for Chester Chipmunk
The player can visit Creptmag's Obvervatory in Trevor 2. A spirit appearing to the player in Angler's Fortress warns them that Angler is planning to utilize ancient magic discovered by Creptmag; it is unclear if this is the spirit of Creptmag or not.
Trevor 3: Shadow of The Chipmunk
While not appearing directly, Creptmag is brought up several times by various characters. Eventually, the player learns that the enchanted amulets were forged on North Island due to an abundance of surface level bontonite deposits "eons ago."