Aquamentus is a boss character in The Legend of Zelda for the NES. He is a green, colossal creature who has the appearance of a unicorn and dragon. He attacks by spewing three fireballs at once towards his opponents, and the only way to kill it is to attack its horn which is featured on top of its head.
It appears that Aquamentus is a species, rather than a specific character, seeing that two of them appear in The Legend of Zelda, where they guard two different labyrinths - the eagle and lion ones.
Also, the guardian of the Gnarled Root Dungeon in The Legend of Zelda: Oracle of Seasons happens to be an Aquamentus.
In the first Legend of Zelda, if you Link has all his hearts at the end of the dungeon up to the boss, you can stay safely in one area out of range of the beasts fireballs and shoot your sword at Aquamentus until it is defeated, which is a much safer way to beat the dungeon.