Tail Key

The is an item in.

Location and Uses
The Tail Key is found in a Treasure Chests located in the Mysterious Woods. At first, Link can not access it, as the forest is looping over itself due to Tarin being transformed into a Raccoon. Link must sprinkle Magic Powder on Tarin to return him to his human form, thus preventing the forest to loop and being able to access the Treasure Chest located to the north.

It allows Link, by inserting it into the Tale Keyhole, to open the Tail Cave's gate.

Nomenclature
The Japanese name of Moldorm, the Boss of the Tail Cave, is Degu Tail, which may explain the name of this item.