The Devolution Gun (or Devo Gun for short) is a powerful weapon introduced in the 1993 Super Mario Bros. film, used and designed by President Koopa. The Devolution Gun prop seen in the movie was a Super Scope, except that it was repainted with metallic paint.
The device contains the powers of de-evolution invented by President Koopa to regress creatures back to their ancestral species. For example, it can transform humans into primates, having been designed specifically as weapons against the Mammalian dimension. Hitting a Dinohattanite denizen will result in the person de-evolving into their ancestral species, either a dinosaur, reptilian form or primordial slime. Other inventions by President Koopa with de-evolution technologies include the Devo Chamber and Devo 4, full room-size machines capable of said transformations.
Super Mario Bros. film
In the Super Mario Bros. film, President Koopa aims it at Mario and Luigi in attempts to stop them from trying to save Daisy and thus halt his plans of conquest, although he wasn't able to fire at them. Koopa, upon the merging of the Dinohatten and Earth dimensions, shot it towards Mario, who dodged it. The shot hits Anthony Scapelli who regresses to a chimpanzee, much to Koopa's amusement. Koopa later attempts to attack Mario with the Devolution Gun again, which Mario deflects the blast with a Mushroom given to him by The King's fungus form. Mario and Luigi are able to get grasp on Devolution Guns of their own, which they use to shoot at President Koopa and devolve him into a Tyrannosaurus Rex. Mario and Luigi continue their continued shoots towards Koopa until he regresses into primordial slime which oozes down to the streets of Koopa Square.
Super Mario Makai Teikoku no Megami
In the manga adaptation of the Super Mario Bros. film titled Super Mario Makai Teikoku no Megami, Koopa introduces his de-evolution technologies by turning Spike and Iggy into Goombas. Koopa and his Goomba army bring their Devolution Guns to Earth during the dimensions' brief merge, which they use to turn humans into monkeys. The dimensions re-split, and Mario begins pelting Koopa with Exploding Mushrooms, something Koopa scoffs at telling him to aim for the head instead (as the manga's depiction of Koopa has regeneration abilities). As Koopa's arm is dismembered, Mario reveals the aim was intentional as he takes the Devolution Gun away from his severed arm and uses it to de-evolve Koopa into fungus.
In the 5th chapter of the 9th volume of Super Mario-kun, Bowser resorts to use the Devolution Gun on the heroes, transforming Luigi into a monkey and Yoshi into nothing since "Yoshis never evolved," and Mario manages to protect himself using the Tanooki Suit and its statue form. After defeating him and destroying the gun, Luigi becomes human again.