In Ocarina of Time, the Keaton Mask is the first the player will come in possession of. It has been generally accepted that it is based around the Pokémon known as Pikachu, which was and still is the most popular species of Pokémon. It could be traded with the guard in Kakariko Village for a small profit, though then the player can head back to the Happy Mask Salesman for another mask. In this game they claim that Keaton is a popular creature among children, similar to Pikachu.
In The Legend of Zelda: Majora's Mask, the Keaton is represented quite literally as a real life species, and is seen as fox like creature with three tails. The player is not requested to see the species, though if they desire to they need to have the Keaton Mask on and cut the grass in Clock Town. By answering their questions correctly they'll give you a Piece of Heart.
It would take quite some time until the species appeared once more, and the game in which they appeared in, The Legend of Zelda: The Minish Cap, they were enemies rather than allies.