Its body surface is always wet and slick with an oily fluid. Because of this greasy covering, it can easily slip and slide out of the clutches of any enemy in battle.
(From Bulbapedia) Poliwhirl is a blue, bipedal Pokémon with a spherical body. It has bulbous eyes that protrude from the top of its head. A black and white swirl covers its belly, which are actually its organs showing through its thin skin. It appears to have white gloves on its hands and rounded, digit-less feet.
While Poliwhirl is capable of moving on land, it prefers to stay in freshwater ponds and lakes. When out of water, it sweats to keep its skin moist. It is always covered in an oily fluid that allows it to slip away from enemies. All of its skin feels this way except where its swirl is, which is very smooth. The swirl on its belly undulates slowly, and staring at it induces drowsiness.
(From Bulbapedia) Poliwhirl is based on a glass frog. The spiral pattern on its belly is supposed to represent how the organs of these frogs and other tadpoles can sometimes be seen through their transparent skin. Its evolution is based on a tadpole's metamorphosis.
(From Bulbapedia) Poliwhirl may be a combination of polliwog and whirl (referring to the spiral on its stomach).