Saddled bichir

The saddled bichir (エンドリケリー, Endorikerī) is a fish that can be found in the river from June until September from 9 p.m. to 4 a.m.. It has a large shadow and can be sold for 4000 bells.

Donating in New Leaf
"The cool-looking saddled bichir are ancient fish with spiky fins on their backs. Though they've been around for more than 400 million years, they remain relatively unchanged. They have big appetites and will wait to ambush any fish they think will fit in their mouths. They can grow up to 30 inches (76 centimeters) long, making them rather large fish indeed. They swim slowly in a way that gives them the appearance of walking like dinosaurs."

Further information
The Polypterus endlicheri endlicheri is a predatory fish that will consume anything small enough to eat whole or in smaller pieces. Though it can reach great speeds in bursts, it cannot maintain a speed fast enough to keep up with the average fish.