Sumo Bro

Sumo Bros. (or Sumo Brothers) are a sub-species of Hammer Bros. that appear as enemies to Mario. Their first appearance is in Super Mario World as somewhat uncommon enemies that can rain down fire on Mario's head by stomping.

Super Mario World
In Super Mario World, Sumo Bros. are large and orange in color and wear black shells. They also have a spike on top of their heads, meaning Mario or Luigi will be injured if they jump on one. They stand on top of blocks and attack Mario by stomping their foot (the sumo movement known as shiko), which causes a spark to drop below; this spark ignites when it hits the ground, making a wall of fire. Sumo Bros. can only be defeated by throwing a Shell at them, by hitting them with a Cape as Cape Mario or by touching them with Mario when he is powered-up by a Starman. Hitting the block they are standing on from below also works but is more difficult than other means due to the fires caused by their lightning-stomp attack.

New Super Mario Bros. U
Sumo Bros. return in New Super Mario Bros. U, presumably with the same abilities they had in Super Mario World (Except they make eletric waves instead of fire waves). There now seems to be a larger Sumo Bro. with a wrapped towel on his shoulders who is only encountered as a boss in Rock-Candy Mines in the second Fortress replacing Boom Boom.

Super Mario World television series
A single Sumo Bro., known as a Fire Sumo, appears in the Super Mario World television series episode "Mama Luigi". This Fire Sumo appears to attack Luigi as he is exploring the abandoned Dome City, stomping the ground beneath Luigi hard enough to cause it to crack, sending the plumber plummeting downward.

Nintendo Adventure Books
In Unjust Desserts, some Sumo Bros. shrunken by Magikoopa are eaten by Yoshi, and two confront Mario in a tunnel inside the dinosaur. Mario defeats the two by jumping out of the way when they charge at him, causing them to crash into each other and become knocked out. If Mario smashes a Blue Shell thrown at him by a Koopa Troopa, the Sumo Bros. will awaken, but before they can renew their assault, they are sent tumbling down a passageway when the newly Winged Yoshi does a loop-the-loop. Later, some more Sumo Bros. try to attack Mario when he and Bowser's minions are forcibly ejected from Yoshi's body, but because they are still tiny, they are scared off by the seemingly giant Yoshi stomping around.

In Brain Drain, a Sumo Bro. will swap minds with Wooster if he hears the static emanated by Iggy Koopa's Synapse Switcher play on a hot dog stand's radio. After unsuccessfully trying to attack both Mario and Luigi, the Sumo Bro. storms off in a huff over its weak new body. Later in the book, Luigi can encounter some more Sumo Bros. if he decides to head out on his own to look for Iggy and if he manages to steal several cans of Fungus Up cola from the Synapse Switcher in the WMUSH radio station.

Trivia

 * The Super Mario World artwork of the Sumo Bros. lacks the spike on their heads. However, the spike is present in the games and in the cartoon.
 * Although they do not appear in New Super Mario Bros., their name is mistakenly applied to an unrelated species of enemy known as Sledge Bros.