Magmaargh

Magmaarghs are a sub-species of Blarggs that first appeared in Super Mario Galaxy 2. They appear as large molten lava creatures with a huge mouth that attack by rising up from lava pools and biting down at anything in front of them. Like their relative the Magmaw, they seem to be mostly liquid, as they can pass through solid objects.

Super Mario Galaxy 2
They mostly appear in Melty Monster Galaxy, being the titular monster. They also appear in Fleet Glide Galaxy and Bowser's Galaxy Generator. Magmaarghs do not move very much, mostly staying under the lava, arising to chomp down on anything in front of them. If Mario or Luigi gets chomped he loses one wedge of health. In the Melty Monster Galaxy, they appear mostly where there are Pull Stars. In Bowser's Galaxy Generator, 3 of them appear during the Snake Block section. Here, the Snake Blocks split into 2 paths, where there is one Magmaargh per path and when the Snake Blocks come back together, one last one appears, where Mario (on Yoshi) must swallow a nearby Blimp Fruit to pass over the monstrous creature (or he needs to go out of the way onto one of the four platforms if he already lost it).

Super Mario 3D Land
Magmaarghs later return as enemies in Super Mario 3D Land, they appear in the lava pools of Castle levels and will try to bite down when Mario passes by. In this game they are notably smaller than in their first appearance. Even while invincible, they cannot be defeated. A sand variation exists that follows Mario or Luigi in the ground and pops out eventually. These are called Sandmaarghs.

New Super Mario Bros. U
Magmaarghs will be appearing in the upcoming Wii U title, New Super Mario Bros. U. So far, they're found during the battle against Iggy Koopa.