From Nookipedia, the Animal Crossing wiki
The football fish (チョウチンアンコウ, Chōchin'ankō) is an uncommon fish. It can be found between 4 pm and 9 am between the months of November and March.
Donating to the Museum
In Wild World
In City Folk
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the football fish has been caught, the following information can be found in the fish menu:
The Himantolophus or 'Football Fish' is a type of 'anglerfish' found in tropical and sub-tropical waters in the Atlantic, Indian and Pacific oceans. They live deep underwater. Both genders have a gelatinous flesh which thickens in the females, the bacteria on their body also gives them a "glowing" appearance. Females are a lot bigger than males and they also have an esca (which is the growth coming from their head) which they use as a lure to catch other fish. The males lose their digestive system as they grow older, so therefore need to find a female before they die. Once they have found a female, they undergo a peculiar mating ceremony, where the male bites and latches onto the female and deteriorates while it is still attached to the female, once the mating is complete. Hence, only adult female football fish are ever found. The males and females were once thought to be two different species.