Unicorn Fountain

The Unicorn Fountain does not exist in any of the canonical Legend of Zelda games. It was an element in the unreleased Ura Zelda for the Nintendo 64DD. It is said that this fountain contains the Triforce and was also where Link would learn the Blade Beam attack. Possibly it was in the beta version of The Legend of Zelda: Ocarina of Time but it was removed.

Fountain Entrance
Its believed that the entrance was found in Zora's Domain in The Legend of Zelda: Ocarina of Time before the Great Cataclysm (before Ganondorf took the Triforce), possibly in a cavern too deep underwater for young Link to reach (and inaccessible for adult Link due to the water being frozen).

The entrance has been reached by hacking the game or utilizing a glitch, but the entrance is just a normal wall.