Ghetsis' Hydreigon. Some people had trouble with it, but some were successful in defeating it by luck or strong team or both. But, N (who is also fully called Natural Harmonia Gropius) wasn't the only one mistreated. Due to Ghetsis being rude to his Zweilous, after many failed attempts, Zweilous started hating Ghetsis more and more. Ghetsis forced him too much, but Zweilous gave up and listened to Ghetsis, though he still hated him. Ghetsis' Hydreigon even tried to kill his trainer but was unsuccessful.
However, afterward, Ghetsis loses against the trainer, making his plans for world domination and Pokémon Liberation unsuccessful, and Hydreigon started hating him more. He was the cause of Ghetsis' right eye. He broke through his eyepiece and then bit his eye, and it started bleeding. Ghetsis got a new eyepiece after this. But, this isn't all. Back then after Zweilous' forced evolution, Hydreigon not only hated Ghetsis but, almost everyone and every Pokémon but, before Ghetsis even appeared in games, his Hydreigon was a merciless murderer.
It murdered lots of Pokémon and trainers with its attacks and almost destroyed everything. After that, a lot of people were against Ghetsis and his only Pokémon, Hydreigon. It killed almost every one of them but, then there was a single trainer who wasn't Hilbert/Black neither Hilda/White who stood a chance against Ghetsis and his almost unbeatable Hydreigon.
Hydreigon fainted but, it was not ordinary fainting, Hydreigon bled and almost was dying. Ghetsis, who did not care for Pokémon that had deep hatred for him, abused it, making his Pokemon only hate him more and more, Ghetsis was seen abusing his Pokémon, almost killing it, and he was forced to stop.
One person used max revive on his Hydreigon, healing it from wound and near-death experience. Not only Hydreigon was murderer but, after Ghetsis got new Pokémon (Before being defeated: Cofagrigus, Bouffalant, Bisharp, Seismitoad, Eelektross After being defeated: Replaced his Bisharp and Bouffalant with Drapion and Toxicroak) they used their attacks NOT to attack Pokémon but to try to kill them but, due to killing Pokémon being impossible in Pokémon games, they can only faint. (Except Ghost Marowak but we weren't talking about her.)
