I'd like to finish some achievements, but my pet is full. Is there something I can to to make him hungry again? A game, an item, a local?

Using in battle with a different pet should randomly make your not-in-battle pets hungry, but it's hit or miss, and a tier 6 weapon. That's the only way, other than just waiting though, unfortunately

Thank you! I'll no longer aimlessly wander the countryside looking for some appetite inducing activity or item :D

Honestly ... All of this time ... I've been training Cibophobia to get to T6 because I thought using that would make the pet battling with it hungry. I mean, not that I'm sad that I have a second mid-tier battle pet now, but wow. Thank you for enlightening me.

Oh no :( Honestly, I've never tested it myself, but that's the way I've heard it explained