From the Super Mario Wiki
A Heat Gun is a handheld weapon which capable of firing a blast of red hot energy. Only one was ever seen in The Super Mario Bros. Super Show! episode "Star Koopa", where it was briefly used by Mouser at one point.
Darth Koopa, having frozen Mario, Luigi, Princess Toadstool and Toad with his Freeze Gun, brought them aboard Koop Star as his captives. As Darth Koopa needed to negotiate with Princess Toadstool, he ordered Mouser to defrost the heroes using his Heat Gun.