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 underworld and the dragon will start shooting beams at him. The key to defeating Aquamentus is to attack with your sword when he lands. Once Link defeats the dragon, the first shard of the Triforce of Wisdom is his.

Aquamentus is also the boss of the seventh underworld. 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 cameo appearance in the Game Boy Color game, Oracle of Seasons.

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