Trinexx

Trinexx (デグロック, Degurokku, "DEGROCK") is the boss of Turtle Rock in A Link to the Past and the last dungeon in Ancient Stone Tablets. Its name may be derived from the prefix "tri-" meaning three, and the word "necks," alluding to its three heads. Its first form is a stone turtle with two additional heads protruding from its shell on either side. These extra heads are elemental; the red one spouts flame and the blue one breathes frost.

Strategy
In order to defeat Trinexx, Link must use the Fire Rod on the blue head and the Ice Rod on the red head to stun them. Because of this, it is recommended to bring blue or green potions. After stunning the heads, Link can attack them with the sword to destroy them. It is prudent to destroy the ice head first, as its breath creates a streak of ice across the floor which will cause the hero to lose traction. While attacking, Link must dodge its main head which lashes out at him. After both extra heads are gone, Trinexx then withdraws into itself and its shell will explode, revealing its true form as a giant snake with a glowing orb in the center of its body. In this form, Trinexx attacks Link directly with its body. To finish the boss, Link must strike the orb three times. Once Trinexx is defeated, Link receives another Heart Container and the crystal containing Princess Zelda.

Trivia

 * Turtle Head, the mini-boss of the Turtle Rock in Link's Awakening, is very similar to Trinexx and was likely inspired by it.
 * In the A Link to the Past comic, the monster Link fights in Turtle Rock is a three-headed dragon, though its body is never shown. As with Trinexx, one of its three heads breaths fire and another breaths ice, but the dragon is never actually called "Trinexx".
 * In the A Link to the Past manga, Agahnim kidnaps Ghanti and turns her into Trinexx with his dark magic. He forces her to fight Link, who doesn't realize Trinexx's true identity until he strikes her down.
 * Due to the nature of Trinexx's elemental attributes, it could be speculated that Trinexx is the form taken by Twinrova upon entering the Dark Realm.