Tadpole

The tadpole (オタマシャクシ, Otamashakushi) 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 unknown what species of frog the tadpoles are, but they are most likely the same species the frogs are.

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