Aquamentus

Aquamentuses are large, green dragons who appear in a few The Legend of Zelda games. They have a horn on their forehead, making them resemble a unicorn.

The Legend of Zelda
In The Legend of Zelda, Aquamentuses appear as the bosses of the first and seventh labyrinths, Eagle and Demon. The first Aquamentus serves as the first boss of the series overall.

Aquamentus appear with their backs to a door, with the near corners of the room covered with a large amount of blocks. They only ever face to the side, and walk slowly forward and backward a short distance, occasionally firing off three projectiles simultaneously. Their horn is their weak point; attacking it enough will destroy the creature.

In the Second Quest, Aquamentuses appear as enemies in Level-4 and Level-8.

The Legend of Zelda: Oracle of Seasons
Like the other bosses from the first game, an Aquamentus reappears in The Legend of Zelda: Oracle of Seasons, where it is the boss of the Gnarled Root Dungeon, itself heavily based on Eagle and also the first dungeon. In this game, its wings are larger and it is capable of flight. At the beginning of the battle, it slowly flaps down into position. It attacks roughly the same as before, though it sometimes uses its horn to charge forward due to the room being larger. After charging, it flies back into position. Its horn is now a separate graphic from the rest of the creature, and it blinks red. When attacked enough, the horn disappears entirely while the rest of the Aquamentus explodes the same as the game's other bosses.

The Legend of Zelda Game Watch
In The Legend of Zelda Game Watch, an Aquamentus (referred to generically as a dragon) acts as the boss of each of the four caves. Each must be defeated with the bomb found in the cave.

The Legend of Zelda (television series)
In The Legend of Zelda by DiC Entertainment, an Aquamentus appears in Underworld Connections. It is far more horse-like than in the game. It is also an aquatic creature, likely due to its name, though it may also tie it in with another type of mythical horse, the. It guards a Triforce shard found under the Black Falls.