In the anime episode 72 - 'Go West Young Meowth', you get to see a flashback in which details Meowth's history and how he learned to talk. He was a stray who, when hungry, attempted to steal food and was not very successful. He came across a gang of Meowth, led by a wild Persian who gave him food and shelter so he decided to join this gang. One day he came across a trainer-owned Meowth by the name of Meowsy, whose trainer was extremely rich. He attempted to woo Meowsy but was rejected for being wild and dirty. He decided then, to be more like a human since Meowsy had a trainer, and learned to talk and walk upright by himself. He presented himself to Meowsy who rejected him for being a freak. He vowed since that day to become richer than that trainer in hopes to win her heart and then joined Team Rocket soon after.