Fin Piranhas are an recurring aquatic enemy featured in the Zelda series. They lurk underwater, and are vulnerable when they jump out. A simple sword slash will easily defeat them.
This type of enemy can be encountered within Angler's Tunnel and Goponga Swamp in addition to various outlying caves. They can be easily located with their fins sticking our of the water, occasionally jumping upwards. This makes them vulnerable, and this is the only opportunity to attack them.
Oracle of Seasons
In Oracle of Seasons, Fin Piranhas can be found to the north of Sunken City and Eyeglass Lake. Like typically would, these enemies will only appear in places occupied by water. Just like in Link's Awakening, they're vulnerable to attacks when they spring out from the water.