What could be considered a normal behaviour in a duskull? I’ve recently caught one and while usually pretty calm, it likes to tug at my hair and prank my other pokemon a bit too often. Nothing harmful or serious, just childish. It does obeys commands in battle, so I guess it’s just excited, but I’m not really sure.

That actually sounds like normal behavior for any ghost-type, especially one that may have had a misdreavus or shuppet father. Duskull especially feed off fear and surprise, so there’s also the possibility that he’s attempting to get a between-meal snack. You may want to increase the amount of kibble he receives to tide him over from one meal to the next or otherwise work hard to establish the fact that you will, in truth, take care of all his needs (which will hopefully discourage him from taking care of itself).

However, in some varieties of duskull, this is actually their form of affection, so if the above doesn’t work, show him methods of affection you do approve of and use positive reinforcement (in the form of treats—duskull typically like sweets) whenever he engages in them.