Gleerok

Gleerok is the guardian of the Cave of Flames high atop Mount Crenel in. It is the second major boss of the game, and it guards the Fire Element.

Characteristics
Defeating the Gleerok involves firing the Cane of Pacci at its back, and then climbing on top of its neck and slashing the yellow thorn on its back with Link's Sword while it is stunned. Gleerok's main form of attack is to spew fire on any surrounding prey or threats. If all else fails, it will submerge itself in its lava pool and cause major disruptions within its abode, including an increase in lava flow and tectonic disruptions that cause chunks of rock to fall from the ceiling. Gleerok is most likely an evolutionary offshoot of the Gleeok, or a mutant/mutated Gleerok, adapted to harsh lava-filled environs. Along with this evolution/adaptation, it seems to have lost the ability to sprout multiple heads.

(Himekawa)
In the, Gleerok was, prior to the battle, cursed by Vaati. Once defeated, Gleerok turns back to its true form, a softer, plesiosaur-like creature. The newly transformed Gleerok then gives Link the Fire Element.