Explore Koholint deeply enough in The Legend of Zelda: Link's Awakening and eventually you'll find a telephone that only calls one number... and it's a wrong number at that.  While every other phone on the island calls the old hint-giving Grandpa Ulrira, using the phone inside Ulrira's instead calls an unusual creature that calls itself the Bucket Mouse.  Who or what is the Bucket Mouse and what does it have to do with Link's quest?  It turns out that it's an incorrectly translated in-joke from the Japanese development team.  Glitterberri's Game Translations has an interview with the designers from years ago and one of them, Kazuaki Morita, spills the beans.

The "Yeeees!  This is Bucketmouth!!" you hear when you make a call from Ulrira's house is a real fishing shop, and the owner is actually the guy who runs the fishing pond in Ocarina of Time.

Translator’s Note: The contextless “Bucketmouth” was incorrectly romanized as “Bucket Mouse” in the English translation of Link’s Awakening. The line “Yeees! It’s Bucket Mouse!” should be “Yeees! You’ve reached Bucketmouth!” The actual shop in the Neyagawa district of Osaka, Japan allegedly closed its doors in October, 2010.

So that explains it!  Personally, I prefer the idea of a mouse that lives in a bucket hidden away somewhere on the island rather than a fishing shop proprietor.  You can't find Bucketmouth or the Bucket Mouse anywhere in the game, of course.  It's just a gag thrown in for the team's own amusement.  Still, it's great to put that mystery to rest after all these years.  It may well be Koholint's last lingering mystery.