Tadpole

The Tadpole (オタマジャクシ, Otamajakushi) is a fish (technically an amphibian) introduced in. It lives in ponds, sharing its spot with Crawfish, Killifish, and Frogs. They sell for only 100 Bells, making them the cheapest fish in the game. It is unspecified what species of frog the Tadpoles are, but they are assumed to be the same species as the Frogs are.

In
Upon donation to the museum, the information board in front of the tank that houses the Tadpole says the following:

In
The Tadpole can be found in the open, recessed tank in the freshwater fish room in the museum, beneath the Frog.

Real-world information
Tadpoles are frogs in their larval stage. They have no arms or legs until adulthood and usually live underwater.