Aquamentus

The Legend of Zelda
Aquamentus (アクオメンタス, Akuomentasu) is the first boss in the The Legend of Zelda, appearing in both the first and seventh labyrinths.

If the player rushes into the first labyrinth unprepared, this dragon can be a little challenging. If rupees are saved up for a shield first, then this enemy becomes a snap. 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 the boss of the first labyrinth once again, this time known as Gnarled Root Dungeon. Unlike in his first appearance, the player 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 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.

Coincidentally, both Eagle Labyrinth and Gnarled Root Dungeon are nearly identical in terms of the design dungeon rooms create when viewed from above.