Dark Type Pokemon

Dark Type Pokemon are the best barriers against Psychic types because psychic type moves are useless against Dark Types. Dark Type moves are also super effective against Psychic types and Ghost types. But as all types, dark types have weaknesses, notably the bug and fighting types. So don't switch a dark type when you're facing a bug types and watch out for those fighting moves carries by non-fighting types!

Murkrow's up first and Murkrow is fast. Murkrow can fly and can learn Mean Look, which will stop those running Pokemon from running away! Murkrow does awesome in Cool Contests and can be found in the cave on five island(I think). The cave is to the north of one of the islands and the island is also where rich Lady Selphy lives. The best way to catch Murkrow is to take a Pokemon that knows Sweet Scent and Murkrow will eventually come out after a few times.

Sneasel is exclusive to Pokemon LeafGreen and can only be found in Icefall Cave on Four Island. Sneasel is extremely rare and is a dark and ice type, meaning it has a 4x weakness against fighting types.

Houndour can be found in Pokemon XD and can quickly evolve into Houndoom, a very fast Pokemon. It is a Fire and Dark type and can also be taught Solarbeam! Flamethrower and Crunch are essentials for this Pokemon.

Absol is a very rare Pokemon that can be found to the right of Fortree City in Pokemon Ruby/Sapphire/Emerald. It can learn Slash and is fast and strong. Beware of Fighting moves because this Pokemon lacks in Defense! (It seems Absol is very hard to find. Once the English version of Pokemon Diamond/Pearl comes out, maybe Manjyome will be willing to trade one for one of your Pokemon.)