[header] How to get a hydreigon in pokémon black [title] Go to victory road. [step] You must first catch a deino, which can be found on the 1st and 3rd floors of victory road by walking, and it has a 20% of showing up. The location of deino is the same in both black and white. 
[title] Carry a pokémon with you that has status-affecting moves such as thunder wave or hypnosis. [step] It would also be wise to have a pokémon who knows the move false swipe.