Fish

These fish can be captured in bottles in the lands of Termina and Hyrule. They are needed to gain entrance to Lord Jabu-Jabu's belly in Orcarina of Time. It seems that despite that these Fish are easy to find, they are can be quite valueable if you're looking to get some easy money. However, only certain people will offer to trade with you.

Fish can also be captured in A Link to the Past when the water is drained out of a certain pond in the south of the Light World. If this fish is thrown back into the water it will say "thank you" and throw you some rupees.

In Twilight Princess a fish has to be caught at the beginning which a cat then steals and runs off into the shop, this then unlocks the slingshot in that shop.

Also later on a fish called the Reekfish needs to be caught using a Coral Lure in Zora’s Domain.