A polywog is a tadpole, which is a baby frog or toad. There are many different kinds and sizes. They look like little minnow fish, but gradually grow their arms and legs and look like adult frogs and toads.
Polywog is simply another word for tadpole, a baby frog. The word is an alteration of Middle English word polwygle that itself is a fusion of two words meaning poll and wiggle.