Aquamentus

Aquamentus in The Legend of Zelda
The first boss ever in the Legend of Zelda series, Aquamentus is a large dragon.

Link enters the boss room of the first Dungeon and the dragon will start shooting fireballs at him. The key to defeating Aquamentus is to attack his head with your sword while avoiding the trio of fireballs he periodically shoots at you. Once Link defeats the dragon, the first shard of the Triforce of Wisdom is his.

Aquamentus is also the boss of the seventh Dungeon. The strategy is the same. The seventh piece of the Triforce of Wisdom is your reward for his second defeat.

Aquamentus in The Legend of Zelda: Oracle of Seasons
Aquamentus makes a second appearance in the Game Boy Color game, Oracle of Seasons.

Once again, he is the boss of the first dungeon you encounter. He is defeated the same way as in the first game, with swings of sword to his head while he is on the ground. Upon his defeat, Link receives the first Essence of Nature, the Fertile Soil.