Trinexx

Trinexx (デグロック, Degurokku) 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 (it is recommended to bring either blue or green potions before engaging this boss), Link must use the Fire Rod on the blue head and the Ice Rod on the red head to stun them. Link can then attack them with the sword to destroy them (it is helpful for him to destroy the blue head first, since its frost breath can create a permanent streak of ice on the floor that worsens motion). While doing this, 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 it attacks Link directly with its body. Hitting the orb three times defeats this boss to win the dungeon's crystal and Heart Container.

Trivia

 * 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".
 * Turtle Head, the mini-boss of the Turtle Rock in Link's Awakening, is reminiscent of Trinexx.