Poliwhirl is a portmanteau of polliwog and whirl. Whirl refers to the spiral on its stomach, while Poli is also the "family" name for this evolution chain. The Japanese version of the name is probably onomatopoeic: "Nyoro nyoro" describes "wiggling" or "squirming" or "slithering", which is how a tadpole or a "Poliwag", its most basic form, would move. The download can be found here.