Magmaw

Magmaws are a smaller variant of Magmaargh that first appeared in Super Mario Galaxy 2

Super Mario Galaxy 2
In Super Mario Galaxy 2, they are only found in the Melty Monster Galaxy, mainly in The Magnificent Magma Sea level. Magmaws live in lava and attack by opening their mouths and passing through asteroids that the player may be on. They seem to be mostly liquid because small objects such as meteors can pass through their body. Much like their larger counterparts, Magmaarghs, they cannot be defeated.

New Super Mario Bros. U
Magmaws make an appearance in New Super Mario Bros. U in Peach's Castle. They appear in Magma-River Cruise where they come up from the lava, move towards the player, and go back down.

New Super Luigi U
Magmaws return in New Super Luigi U where they appear in the levels Magmaw River Cruise and The Final Battle acting the same way as in New Super Mario Bros. U. Magmaws, Cheep Chomps, and Magmaarghs are the only enemies who can defeat Nabbit.