J briefly had a Hydreigon in Unova - it was quite infamous in the area he found it in for its unruly and vicious temperament. Even when trying to appeal to it, the Hydreigon would lash out against J's other Pokémon and even try to attack him. It came to light very quickly that the Pokémon simply did not wish to live its life alongside a trainer, and was best suited to living in the wild. J was quick to accept this, and released it back where he caught it originally. In terms of Pokémon that he still has, his Froslass certainly took a lot of time to train. J was very patient with Naoko and made it clear to her that he was prepared to wait for as long as she needed to feel comfortable with proper training, and eventually she came around. She's now one of J's most formidable and reliable Pokémon partners :)