Rex Mars Atomic Discombobulator is a toy ray gun that is for sale at Cockamamie's. It costs fifty dollars.
When John came to take the Simpson family for the ride, he pretended to zap Homer with the gun.
John later gave the gun to Bart who pretended to zap Homer, too. Homer who thought Bart was becoming gay because of John's influence, became worried even more when he saw Bart doing it.