From the Super Mario Wiki
A Freeze Gun is a handheld weapon capable of encasing its target in a shell of ice. Although its power is unknown, it is fully capable of freezing a human. Darth Koopa and Mouser are the only known characters to have ever had a Freeze Gun.
Freeze Guns were only ever seen in The Super Mario Bros. Super Show! episode "Star Koopa". Darth Koopa, who appeared to have been seemingly defeated by Mario in a Lightplunger duel, pulled out his own Freeze Gun at the last second, and used it to freeze the plumber. Princess Toadstool demanded for Darth Koopa to defrost Mario, but naturally he froze her, along with Luigi and Toad.
After bringing Mario's group aboard Koop Star, Darth Koopa ordered Mouser to defrost the heroes using a Heat Gun. When Mario attempted to fight back, Mouser threatened to permanently freeze Mario with his Freeze Gun, convincing him to stand in one spot.