Oarfish

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

In New Leaf
The information board in front of the tank that holds the oarfish says the following:
 * "Oarfish are deep-sea dwelling fish with long, thin, silver bodies and one red fin lining their backs. They can reach lengths of over 10 yards, making them huge and striking fish. They usually swim by twisting their long, thin bodies, but they can also swim in more vertical positions. Their mouths are small compared to their bodies and they have no teeth, so they're large but not imposing."

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