Goomboss (originally known as the Goomba King) is a character that first appeared in the game Paper Mario for the Nintendo 64. Like his title suggests, he is the king or ruler over the Goombas.
After his boss appearance in Paper Mario, Nintendo put him in the Super Mario 64 remake, Super Mario 64 DS for the Nintendo DS, where he was renamed Goomboss. He wasn't in the original, but when they remade the game, they added new courses, and Goomboss was the boss in the level in Peach's room. He was found once you jumped inside a huge tree bark, where you entered his lair. Here, there was a huge circle that you'd run around, with poison water in the middle, the player's character would have to throw a Goomba at Goomboss three times to kill him.
He appeared later in Mario Kart DS as a boss in mission 3-9. You must race and beat him on the Baby Park track. He runs around the course throwing Goombas and the occasional Super Mushroom. In the last lap he cheats and cuts off almost half the track.