Eel

The eel is a very special fish in the third and fourth Animal Crossing games because it is the only fish with a long and thin shadow. It is a rather expensive fish, selling for 2,000 Bells at Tom Nook's store. The fish is another summer fish, of which there are many, and appears in the evening and night.

Upon Donation in City Folk
The eel appears in the right tank in the museum, when donated. It can be seen as a brown-coloured line moving across the bottom.

Further Information
The American eel, Anguilla rostrata is a fish that spawns in salt water. After hatching, the baby eels make their way to North America, on the east coast. Teh eels, if hungry enough, may eat their own family to survive the trip to America. They prefer to hunt at night, which is referenced by the time it appears at in Animal Crossing.