Nyarth is a member of Rocket-dan (Team Rocket), more specifically part of a trio with Musashi and Kojirou, that follows Satoshi and his friends around in the Pokémon anime, usually trying to steal Satoshi's Pikachu. His major difference from other Nyarth, and nearly all other Pokémon, is that he can talk and walk on two legs like a human.

(Source: Bulbapedia)