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.
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 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 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. They had also tried to kidnap its mother, severely wounding her. They appeared in person in Mother of all Battles.
Rico, a poacher from the Hoenn region, appeared in A Poached Ego!. He was trying to steal a group of wild Ekans and Koffing, but was later arrested by Officer Jenny.
J is the first recurring poacher. She travels the Sinnoh region with her henchmen, stealing Pokémon on behalf of her clients, who often give her 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 uses sinister gadgets to achieve her goals.
Another Pokémon poacher appeared in The Drifting Snorunt! where he attempted to steal a wild Snorunt. He also battled Ash, Dawn, Jessie and James to achieve this.
Rizzo, a Pokémon hunter from Unova appeared in Guarding the Guardian of the Mountain! where he was hunting down a Volcarona.
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 also had his Pokémon attack Jessie and James.
Notable Pokémon poachers