Jermafenser

Helmethead is the boss of Midoro Palace in Zelda II: The Adventure of Link. He reappears as the boss of the Palace on the Sea in the original Family Computer Disk System version of that game; in the later cart version, he was replaced with Gooma.

Strategy
Like Horsehead, the previous boss, his weak point is his head. Unlike Horsehead, he attacks Link by throwing fireballs at him from his head. When Link lands a blow to the head, the helmet will come off and be replaced by another one. The detached helmet will then begin to float around and attempt to kill Link by shooting more fireballs and running into him. When Link knocks off two helmets, his true head will rise from the armor - a white head (seemingly with one eye). For the rest of the battle, Link fights the boss while also dodging the fireballs shot by the helmets. After achieving victory in the fight, Link returns one of the Crystals to the palace.

The original refight
In the Family Computer Disk System version, the boss was a little easier due to having only one floating helmet. Gaining a second head as well as more strength, Helmethead seeks revenge as the boss of the Palace of the Sea. Since the later cart version gave the two helmets to the first encounter, the second was replaced with a fight versus the newly designed Gooma, a "boss version" of the Guma enemy.

Non-Canon Appearances
Helmethead makes a brief appearance in Captain N: The Game Master during the episode entitled Having a Ball. At the end of the episode, Helmethead stands tall behind the throne at King Hippo and Eggplant Wizard's party in the tower, fanning King Hippo, while Moblins play music in the background. When Link, Kevin and Princess Lana retrieve the Triforce pieces from Eggplant Wizard and King Hippo and use all three against them, Helmethead is destroyed along with the Moblins.

Trivia

 * The Official Nintendo Player's Guide referred to Helmethead as "Jermafenser", a possible transliteration of his Japanese name, although all other media and guides referred to him as Helmethead.
 * Of course, Helmethead is named for his dullahan self. He's quite the fencer, though it's not clear what "Jama Fencer" may mean; it could be "Jermafenser" anyway and/or totally made-up.