Oarfish

The Oarfish (リュウグウノツカイ, Ryūgūnotsukai) is a fish that can be found in the sea between December and May. It first appeared in and can be sold for 9000 Bells.

In
The information board in front of the tank that holds the Oarfish says the following:

In
When donated, the Oarfish can be found in the north-eastern-most room of the fish exhibit, in the "sea" tank which is near the submersible and diving suit. It shares this tank with the Barreleye, Coelacanth, and Football Fish.

In
During a Fishing Tourney, when given a Oarfish, Chip will say the following:

Real-world information
Oarfish are most probably the origin of the sea serpent mythology. The king of herrings species has a published length of 11 meters (36 ft), with smaller members of the family ranging from 4.6 m (15 ft) up. Due to the consistency of their skin, oarfish rarely are caught alive.