From the Super Mario Wiki
Blarggs are large lava dinosaurs commonly found inhabiting pools of lava in underground caverns. Originally, Blarggs resembled lazy eyed lava dinosaurs, but more recent depictions show them as being anything from blob-like beings with large mouths, arms and beady eyes to humanoid, limbed creatures. There are even some species of Blarggs that dwell in water or sand, not lava.
Super Mario series
Super Mario World
Blarggs first appear in Super Mario World as fairly rare enemies, encountered primarily in the Vanilla Dome. When Mario or Luigi approach a Blargg on a Skull Raft, the eyes of the Blargg will be visible before they dip back into the lava, moments before lunging. Blarggs can not be defeated by any conventional means for the most part and can only be avoided, though Yoshi is capable of jumping on them and, with some difficulty, eating them. The player can also opt to Spin Jump if lacking a Yoshi, although this method only prevents one from taking damage if landing on a Blargg or its eyes. (Mainly seen and effective in Vanilla Dome 3, also making brief appearances in Vanilla Dome 1 and Chocolate Secret).
Super Mario Galaxy 2
Two new species of Blarggs appear in Super Mario Galaxy 2. The first is the Magmaargh (pictured), which will try to swallow Mario and go back into the lava. The other one is the Magmaw, which will not try to attack directly but will always stay above the lava. When Mario gets touched by them, he loses one wedge from his life meter.
Super Mario 3D Land
Two types of Blarggs appear in Super Mario 3D Land, having similar roles to that of Super Mario Galaxy 2. Although the Magmaarghs return, their sandy sub-species, the Sandmaarghs, make their debut in the game.
New Super Mario Bros. U
Both Magmaarghs and Magmaws from Super Mario Galaxy 2 reappear in New Super Mario Bros. U. The former only appears during the fight with Iggy Koopa, when he will often summon them during his fight against the Mario Bros. and the two Toads via a red spell from his magic wand. Magmaws only appear in Peach's Castle-2: Magma River Cruise. Both retain their Galaxy 2 designs and both attack like a Blargg.
Super Mario 3D World
Super Mario World television series
Blarggs, though never referred to as such onscreen, appeared in two episodes of the Super Mario World animated series.
In the episode "The Wheel Thing", Mario and Luigi, having been banned to the Lava Pits, begin attempting to cook a pizza over a fire. The smell from this cooking pizza causes a Blargg to emerge from a nearby lake of lava and begin to chase Luigi, attempting to eat the plumber. Luigi is rescued from being devoured as Mario swoops down and saves his brother as Cape Mario.
In a later episode titled "Mama Luigi", a Blargg makes a brief appearance. Luigi and Yoshi, while running away from a pair of Tyrannosaurus Rex, use a Blargg's head as a stepping stone to cross a river of lava.
Nintendo Adventure Books
Super Mario Adventures
Several Blarggs make a minor appearance in Nintendo Power's Super Mario Adventures comic, appearing in the lava flooded basement of Bowser's Castle. These Blarggs are used as stepping stones by Mario to reach a Warp Pipe leading to another part of the castle.
Mario & Wario
A Blargg appears in Honō no Dōkutsu, the fifth stage of Mario & Wario. It is nearly identical to its appearance in Super Mario World, but unlike its previous appearance, it is docile and merely part of the scenery.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Blarggs make their first chronological appearance in Super Mario World 2: Yoshi's Island. They have a different appearance; now they look like blob-like creatures with large mouths, and have frowning black eyes with no pupils. They behave the same way as in Super Mario World, by coming out of the lava and biting Yoshi. They can be defeated with Eggs, but reappear after a few seconds; a POW Block, however, can defeat them permanently. There is also a giant version called Gargantua Blargg, which come out of the lava and try to block Yoshi´s path. These games also introduce a species of water-dwelling Gargantua Blargg known as Nep-Enuts.
In Yoshi's Story, Blarggs are once again altered, being larger and given small arms, they appear in only one level, Stage 2-2: Blargg's Boiler. In this game, Blarggs don't harm Yoshi when touched, although Yoshi gets a forceful push from them. While they can't be harmed, Yoshi can throw an egg at one to force it back into the lava. There's also one Blargg which will briefly pursue Yoshi. However, this one isn't affected by Eggs, so the player doesn't have to throw them at it.
Yoshi's Island DS
In Yoshi's Island DS, Blarggs reappear with the same appearance and behavior from Super Mario World 2: Yoshi's Island. Gargantua Blarggs and Nep-Enuts also appear in this game. There is also a new sub-species called Crusher Blarggs, who are larger and can destroy dense rocks.
Captain Toad: Treasure Tracker
Other appearances and references
Blarggs, with their appearance resembling how they appeared in Super Mario World, were originally going to appear in Super Mario 64 as enemies in Lethal Lava Land, Bowser in the Fire Sea, and even Wing Mario Over the Rainbow as they were found in the game's coding. With GameShark codes, the player can make them appear on the level. No reason was given as to why the Blarggs were cut out of the code.
Blarggs are also referenced in both Mario & Luigi: Partners in Time and Super Paper Mario. In the former, they are mentioned by Kylie Koopa, while in the latter, a Sammer Guy named Raging Blargg can be fought.
Names in Other Languages