Aquamentus

Aquamentus is a unicorn-like dragon and the first boss in The Legend of Zelda and Oracle of Seasons. It appears solely in dungeons. Though its size may be intimidating, the strategy required to defeat it is simple: with its head, or rather specifically its horn 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 as a main boss in the Eagle and Demon in the First Quest, and returns as the boss of the Eagle in the Second Quest as well. However, it also appears as a mini-boss in some other dungeons.

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. Since it is unlikely that Link will have the Magical Shield, the best strategy is to watch for his mouth opening to see when he is about to fire these beams so they can be avoided, or attack from a distance with Sword Beams or the Bow if possible.

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. Bombs can inflict much more damage than Link's Wooden Sword, so these explosives can come in handy during the first fight against this dragon if Link has them.

By defeating Aquamentus in a labyrinth that it appears as a boss, 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, a similar dungeon to Eagle Labyrinth. Unlike in his first appearance, Link doesn't have a chance to "buff up" much before fighting this monster. Link can move diagonally and has access to the Spin Attack now, but Aquamentus also has a new attack in the form of a dangerous charge across the screen. After impacting on the far wall, Aquamentus will take to the air using its wings before landing back on the right side of the room and continuing his attack. Since the dragon is larger than it was, it is also slightly harder to hit his horn. Before emitting beams, its horn will glow. When Aquamentus has sustained enough damage, his horn will be removed, his power will wane and he shall be destroyed. Upon the dragon's defeat, Link receives a Heart Container and the first Essence of Nature, Fertile Soil.

The Legend of Zelda Game Watch
Aquamentus is the boss in each of the four dungeons from, but is referred to as simply "dragon". He can only be defeated using a bomb.

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.

Oracle of Seasons Manga
Aquamentus makes an appearance in Oracle of Seasons manga alongside various other enemies and boss monsters from the game, where they attack Link, Ricky and Maple under Twinrova's supervision before they can reach Onox's Castle. They swarm all at once with Aquamentus leading the charge, while Ricky attempts to protect Link by putting him in his pouch and attacking with Tornado Punches. Aquamentus breaths fire, and while Ricky attempts to block the flames with his tornadoes, he is overwhelmed and defeated. It would of been a lost battle if it was not for the appearance of Moosh, who awakes from hibernation at just the right moment to give them a ride to safety at extremely high speed.

Trivia

 * Aquamentus is the first boss to appear in the history of the Zelda series. Despite this, he has only appeared in as many main series games as Digdogger and Patra and is the only boss not to have any related species.
 * Aquamentus is based on a unicorn and a dragon, but given the "aqua" in its name, may also be based on a hippocampus.