Frog

Frogs are recurring animals in The Legend of Zelda series. Most are not enemies, but either help or are helped by Link. They are often singing as part of a frog choir.

Link's Awakening
In Link's Awakening, there is a frog choir lead by Mamu that teach Link the Frog's Song of Soul after paying a large sum of Rupees.

There is also many frogs inside Richard's house, as a reference to his own game, Kaeru no Tame ni Kane wa Naru ('For Frogs the Bell Tolls').

Ocarina of Time / Majora's Mask
These animals have been included in Ocarina of Time and Majora's Mask as mini-quest characters in order to earn both rupees and a Piece of Heart.

Five of these simple yet curiously useful creatures form the "Five Fabulous Frogs" singing group in the Zora's River in Ocarina of Time, and yet another five form the Don Gero's Frog Choir in the mountains of Termina.

Twilight Princess
In Twilight Princess, there are normal-looking frogs in Ordon Village that Link can talk to while in wolf form.