From Bulbapedia, the community-driven Pokémon encyclopedia.
A Pokémon poacher is someone who captures wild Pokémon, sometimes many of the same species at a time, to sell for profit. These people are sometimes referred to as Pokémon hunters, and are known for having exceptionally powerful Pokémon.
In the anime
Poachers have appeared in a number of episodes as well as movies. Team Rocket was the first major organization to steal Pokémon for profit. The first major hunter was also a member of the said team: the Iron-Masked Marauder, the main villain of Celebi: Voice of the Forest. Using a unique Poké Ball known as the Dark Ball, he attempted to capture Celebi. Another poacher also appeared briefly at the beginning of the film.
A group of poachers, the Pokémon Poacher Brothers, appeared in a memory of Ash's Larvitar in Address Unown. They had stolen the Pokémon when it was still in its Egg and had also severely wounded its mother. They appeared in person in Mother of all Battles.
J was the first recurring poacher. She travelled the Sinnoh region with her henchmen, stealing Pokémon on behalf of her clients, who often gave large sums of money in exchange. She first appeared in Mutiny in the Bounty, and was featured again in Ill-Will Hunting, Pokémon Ranger and the Kidnapped Riolu! Parts One and Two, Pillars of Friendship, Unlocking the Red Chain of Events and The Needs of Three. Like the Iron-Masked Marauder, she often used sinister gadgets to achieve her goals.
Other poachers have also played minor roles with some appearing as a Character of the day. These include:
Pokémon Hunters tend to be far more dangerous than other villains in the series as they typically have no qualms about having their Pokémon attack humans directly or using guns (in the case of the Pokémon Poacher Brothers or Rico, who used an FN P90 model gun) to attack their prey. This is most often shown by J, who in Pokémon Ranger and the Kidnapped Riolu! (Part 2) seemed to be trying to kill Ash repeatedly, and Rico who also had his Pokémon attack Jessie and James.
In the manga
A hunter in Pokémon Ruby-Sapphire
In the Pokémon Ruby-Sapphire manga
Red and his Pokémon encountered a hunter in A Visitor From Outer Space?!.