Goombas in the Super Mario Bros film adaptation are human-evolved dinosaurs that have been de-evolved into this form, having a tiny saurian heads on disproportionately large bodies. As a consequence of the method of de-evolution, the Goombas are rendered significantly unintelligent, but nevertheless loyal soldiers of President Koopa. Depending on the person's ancestry, the Goomba's head type varies, showing either a rounded head, a snake-like head, or a raptor-like head.
The process of their de-evolution involves strapping people onto an automatic chair, binding them by the arms to prevent escape. Afterward, when the knob to the time period of the character's ancestors is set, the victim is reeled away before the chair is launched up into a de-evolution device, exposing the head to rays designed to forcibly de-evolve the person's head down to the size indicated. Koopa commonly uses this as a form of punishment.
Toad, an Anti-Koopa singer, ended up being sentenced to become a Goomba in this manner after he was arrested for singing Anti-Koopa songs. It's also implied that this was the fate of anyone within Dinohattan who committed crimes short of de-evolution of Primordial Slime. Koopa himself nearly ended up becoming a Goomba when the Mario Bros. revolted from the prison and escaped, although the process ended prematurely, not exposing him to enough radiation to ensure that he became a Goomba.
Various Goombas were later being outfitted with Devo-Guns in preparation of invading the Mammalian dimension. The Goombas, however, often had a weakness for dancing, which the Mario Bros. later exploited when infiltrating Koopa's Tower to escape from an elevator crowded with them. Toad later exploited this weakness to prevent the Goombas from following Koopa's order to shoot Mario after the dimensions separated once again.