The Devolution Gun is a powerful weapon in the Super Mario Bros. film. It was mainly used by President Koopa. The device could be used to mobily access the raw power of de-evolution into a gun. When it hits a victim, it will turned them into their ancestral species. It can transform humans into primates, having been designed specifically as weapons against the Mammalian dimension. Hitting a denizen of the Dinosaur dimension, however, will result in the person de-evolving into their ancestral species.
It is used in the movie by President Koopa, where he aims it at the Mario Bros., in order to stop Mario and Luigi 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 dimensions, used the Devolution gun on Mario, although it accidentially hit Anthony Scapelli, which he was nonetheless amused at. He later attempted to attack Mario with it again, although Mario knocked it out of his hands when using some fungus to shield himself from the Devolution rays of the gun. It is finally used by the Mario Bros. to devolve the dictatorial President Koopa into a Tyrannosaurus Rex (Koopa's ancestral species). They go even further to where he devolves into primordial ooze.
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 have always been the same," 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.