The mushroom-shaped character in Mario Kart is Toad. He is one of the best-known supporting characters in the Mario franchise and is recognizable by his mushroom-like head, usually shown with a white cap covered in red spots. Toad appears as a racer in multiple Mario Kart games and is often portrayed as a lightweight, speed-focused character.
Toad’s origins go back to the 1985 Nintendo Entertainment System game Super Mario Bros. There, Toad appeared as one of the Mushroom Retainers serving Princess Peach, delivering the famous message that the princess was in another castle. The character later became a playable hero in Super Mario Bros. 2 and a recurring helper throughout the series.
A common mix-up is treating Toad’s head as a hat. Nintendo producer Yoshiaki Koizumi confirmed in 2018 that the mushroom cap is part of Toad’s body. Toad also belongs to a broader species of mushroom people, so not every Toad-like character in Mario Kart is necessarily the same individual.
Toadette, Toad’s pink-capped counterpart, debuted in Mario Kart: Double Dash!! in 2003. Other mushroom-headed racers and characters, including Captain Toad and various colored Toads, expand the species beyond the familiar red-spotted Toad.