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.

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.