Aquamentus

Aquamentus is the first boss in The Legend of Zelda and Oracle of Seasons, appearing in labyrinth-like regions. Though its size may be intimidating, the strategy required to defeat it is virtually nonexistant: With its head as its prime weak spot, Link must only strike it upon the head to vanquish it in both incarnations.

The Legend of Zelda
Aquamentus is fought in the Eagle Labyrinth (both quests) and Demon Labyrinth (first quest only).

If Link rushes into the creature's whereabouts unprepared, it can be a little challenging. If Rupees are saved up for a shield first, then this enemy can be defeated easily. Aquamentus will periodically shoot three fireballs simultaneously at Link, each of which can dish out plenty of damage, but none will even so much as graze the hero if he blocks them with a Magical Shield. Even without the expensive shield, the fireballs can be dodged easily if Link isn't too close.

Aside from its projectiles, the only method this beast has of harming the hero is bumping into him. Considering how slowly the bulky dragon moves, his tackles aren't very threatening. Link can damage Aquamentus by hitting its horn with Arrows, his Sword, or well-placed Bombs. Considering Bombs can inflict much more damage than Link's Wooden Sword, these explosives can come in handy during the first fight against this dragon.

By defeating Aquamentus in either labyrinth, Link will earn a Heart Container and a shard of the Triforce of Wisdom.

Oracle of Seasons
Aquamentus is once again the boss of the first labyrinth, known as Gnarled Root Dungeon. Unlike in his first appearance, Link doesn't have a chance to "buff up" much before fighting this monster. Since Link can move faster and has access to the Spin Attack in Oracle of Seasons however, the fight is much easier. Aquamentus itself seems less able to sustain damage now than it once was. Regardless, the basic method of defeating him is the same: hit his head with a sword. Upon the dragon's defeat, Link receives a Heart Container and the first Essence of Nature, Fertile Soil.

Animated series
An Aquamentus is depicted as a horse-like monster in the "Underworld Connections" episode of the animated series. Just as Link is retrieving a shard of the Triforce of Wisdom while swimming in the Black Falls, an Aquamentus appears out of a dark, underwater cave. The Aquamentus takes him to the surface while Link is riding on top of it, struggling. Princess Zelda then grabs a bomb from Link's pouch and throws it at the Aquamentus, which instantly defeats it.

Trivia

 * Aquamentus is the boss in the four dungeons from The Legend of Zelda Game Watch.
 * Aquamentus is the first boss to appear in the history of the The Legend of Zelda series.