Mattermouth

Mattermouths are enemies that first appear in Super Mario Galaxy 2. They resemble the skulls of Dry Bones, as well as Bone Dragons. Their name appears to be a pun on "motormouth", as well as a reference to dark matter.

Super Mario Galaxy 2
In Super Mario Galaxy 2, Mattermouths are found in places such as Haunty Halls Galaxy and Bowser's Lava Lair. They eat away part of the ground, which regenerates after a few seconds. If Mario falls into the holes in the ground created by the Mattermouths, he loses a life. Mario cannot destroy them, but he can stop them temporarily by stomping on their heads, which turns them around, causing them to begin eating the ground in the opposite direction. If Mario is attacked by a Mattermouth, he loses one wedge of his Health Meter.

Mario Party: Island Tour
Mattermouths reappear in Mario Party: Island Tour in the minigame Tragic Carpet Ride. They eat up the carpet that the players are standing on, while the players try not to fall. Players that touch a Mattermouth are stunned.

Additional names
In the files for Super Mario Galaxy 2, Mattermouths are known as "BakuBaku", which may mean that they were initially based on Cheep Chomp. Additionally, within the game's "ObjNameTable" file, there is a removed "BossBakuBaku" entry, suggesting that there was originally going to be a boss version.