The complete Pokemon RBY pokedex, part 6

Pokémon Name: Poliwag
Type: Water
Classification: Tadpole Pokemon
Pokédex Number: 060
Ability: Damp and Water Absorb
Dream World ability: Swift Swim
Useful Attacks: Hypnosis
Location Found:
Diamond/Pearl: Routes 225, 227 and 228 (Good Rod)
Platinum: Routes 227 and 228 (Surf)
HG/SS: Routes 6, 22, 28, 30, 31, 35, 43, 44, 45 and various other locations; Pokewalker - Beautiful Beach, Blue Lake
B/W: Route 6, Wellspring Cave, Victory Road, Challenger's Cave, Giant Chasm (all via Fishing); Dream World - Pleasant Forest (requires at least 10,000 Dream Points)

As with any budding life form, Poliwag's just getting started. It's fairly weak from the beginning, though it does shake things up by leaning moves from Psychic, Water and Normal types early on. One simple evolution later and you can add Ground and Fighting to that list.

Evolution: To Poliwhirl at 25