From Bulbapedia, the community-driven Pokémon encyclopedia.
The poacher is a one-time character that appeared in The Drifting Snorunt!. This article will refer to him by his title, as he was unnamed in the episode.
The poacher pursued Froslass's child, Snorunt, with his very powerful Glalie. He did this so as to be able to sell Snorunt on the Pokémon black market, as they are highly desired and will fetch top dollar. However, Ash, his friends, and Team Rocket stopped him, and began to battle his Glalie. Although the Glalie defeated Jessie's Yanmega, James's Carnivine and Dawn's Buneary, Ash's Chimchar (after teaming up with the wild Froslass) was able to defeat his Glalie. Afterwards, both the poacher and Glalie were locked in the same cage that the poacher used to trap Snorunt, and was left for Officer Jenny to deal with.