Goombas (also known as Little Goombas) are one of the major species of Mushroom World, and are among Mario and Luigi's most persistent foes. They first appeared in the game Super Mario Bros., and have been a common enemy ever since. Goombas resemble small, brown mushrooms, and are a fungus-based species like Toads, Amanitas, Spooks, and Shroobs. Goombas are physically weak, and aren't much of a threat to adventurers, as a single stomp usually defeats them, although a number of powerful Goomba sub-species have emerged throughout the years. The Goombas, as a collective race, used to be allied with the Mushroom Kingdom. However, many of the Goombas have turned traitorous and joined the Koopa Troop, an organization led by Bowser. While a vast majority of Goombas allied themselves with the Koopa king, others became independent thieves, or even formed peaceful Goomba communities.

(Source: Super Mario Wikia)