He is actually a Godzilla-lookalike who, in HTF Origins, is given the nickname "Sand Monster" as he guards the Pyramid of Koopa in Desert of Koopa. When you first encounter him, you can only run away, as there's no way to damage him. It was until he falls into a lava pool and you gain access to the underground, where his skeletal form can be found there, exposing his only weakness.
During the first encounter, the entire pyramid section becomes an autoscrolling level. He's immune to all the player's attacks in said encounter. During the actual battle or second encounter, his weak point is exposed in the form of a blatantly clear weak spot marked by an oval. After defeating him, he bursts into an explosion.
The skeleton form of Godzilla attacks by shooting flame.