I don't know why but I can't use one of my pets to battle, and he's not involved in any previous battles. It's been doing this for a while now. Any idea why, if it's happening to anyone else, or what I can try to fix it?

Make sure the pet is very happy, bloated, has full health, and check the statistics page. If there is no battle to quit, try the "get unstuck from battle". If that doesn't work, try unequipping/equipping an item and that should help. Otherwise you'll have to file a ticket after that if your pet still will not show up.

Alright thanks, I hadn't thought to try the item idea and I'll check the statistics page also.