Helmasaur King

The Helmasaur King (ジークロック, Jiikurokku) guards the First Crystal in the first dungeon of the Dark World in A Link to the Past. He is a large quadruped creature with a large mask on his head, and a long tail with a deadly looking stinger on the end. He must be defeated before the first Crystal can be acquired and the quest to advance.

Strategy
Before he can be damaged, his mask must be destroyed with the Magic Hammer or the Bombs. The magic hammer is preferred because it is much quicker. The monster fights by spitting fireballs that split into twos and then fours. To avoid those, stay out of their way by memorizing their pattern. It will also attack by swinging it's long tail at you like a whip.

After attacking the helmet on its face enough times, it will shatter to reveal the face of the monster. (Remember, the monster will become much more frantic at this point, so watch your step.) When this is done, a large green emerald on his forehead appears. This can be hit with the arrows or the sword. When struck enough times, he will explode. When defeated, he leaves behind a Heart Container and the First Crystal in which one of the seven maidens is entombed. In the Gameboy Advance version he returns in the Palace of the Four Sword a secret dungeon only available after beating both Link to the Past and Four Swords. In his secret form you HAVE to hit him with arrows and he also sends helmasaur minions after you.