The Unicorn Fountain is a location that was supposed to appear in the unreleased Ura Zelda for the Nintendo 64DD. It is said that this fountain was supposed to contain the Triforce and was also where Link would learn the Sword Beam attack. It is possible that it was featured in the beta build of The Legend of Zelda: Ocarina of Time, but was removed from the release build and subsequent builds.
It is believed that the entrance was supposed to be located beneath the waterfall in Zora's Domain before the Great Cataclysm, possibly in a cavern located too deep beneath the water for child Link to reach (and inaccessible to adult Link due to the water being frozen). The entrance can be accessed by hacking the game or utilizing a glitch, but the gap is just a wall. This could mean that it was either intended to be a hidden entrance designed to be seen only while using the Lens of Truth or just a last minute removal. If this were true, however, this would cause many conflicts with the reports of it having been meant to contain the Triforce; according to legend, the Triforce's resting place is the Sacred Realm, whose entrance is found within the Temple of Time.