Lately my Nidoking has been acting strangely lately. He’s been extremely grumpy and wont listen. I’ve had him for years and raised him from a little Nidoran and this is the first time he’s acted this way. I’ve tried extra training and changed his diet, neither of which have worked. I don’t know what to do anymore, none of my team will come near him anymore and it breaks my heart, is there anything I can do so he’ll go back to his normal self?

Because you’ve raised your nidoking for years, communication may still be an option for you. You may be more in-tune with your nidoking than you may think, in other words, so I would highly recommend sitting down with him and simply asking what may be on his mind. He may not reply to you in Common (few pokémon can), but at the very least, if he feels up to it, he can indicate through signs what he may want.

However, I have a theory. As he’s an older, presumably unpaired nidoking, he may be lonely. That is, he may be at the age when his kind would want to establish a territory and find a mate, but because he’s confined to your home and your team, he may be unable to do so, which may be causing him some frustration. This isn’t to say that you should release him, of course. Rather, consider obtaining a potential mate or taking him to a breeder to … relieve tension, as it were.

As a note, keep in mind that although nidoqueen can’t produce eggs, nidoking like yours will still happily settle down with nidoqueen mates. Nidoking in such pairings don’t seem to mind the inability to produce eggs, although if you do end up getting a nidoqueen, consider using a ditto surrogate with your nidoking if it appears that she’s distressed by the presence of an eggless nest (which is a rare but still possible occurrence).

If your nidoking seems disinterested in mates, take him out to a wide-open space whenever possible, as having an abundance of territory to roam may also lift his spirits.

Good luck, @darkrivermori!