The Mullet is a large red fish, which can be spotted in multiple places around the island - both in mini-games and on locations on the island.
It is seen in the Ice Fishing game as a big red fish that needs a yellow fish on the line to catch. If the player succeeds, they will be greeted by a cutscene of them reeling it in, and getting a coin bonus. It is also required for three of the stamps, one of them being to catch the normal Mullet, another one to catch 15 gray fish and a normal Mullet without loosing worms, and the other one to catch the Rare Mullet.
It's also seen in Aqua Grabber as a catchable treasure. It is first seen in Clam Waters as an obstacle right at the exit of the Giant Clam location, after the player gets the Grand Pearl. It is a hazard due to no available way to catch it without hacking or cheating. However, in Soda Seas, the player is able to catch it by having Fluffy on the player's claw. If the player manages to get the Mullet into the net, they'll be rewarded with a stamp and 1000 coins.