Edit. The game didn't register that I'd given him a healing potion so he had no Health Points. The problem was fixed when I went to the Healer but I lost my healing potion. :(

My pet lost to Dust Bunny. I clicked Game Over then used a healing potion on him. When I tried to fight another opponent I was told I was already in a battle and it took me to the Dust Bunny Game Over Page so I clicked on the Game Over button again. Now his name doesn't show up as eligible for battle and I can't find a way of resetting it so it registers the battle is over.

I checked the Battle Guide which says -

"How do I pull my pet out of a battle? Click on the Challenge an Opponent page. If your pet is in a battle, it will say so at the top of the page, and give you the option to continue the battle or forfeit. Forfeiting will count as a loss."

That option isn't showing at the top of my Challenge an Opponent page.

I'm glad this is kind of resolved. If a healing potion is given when the pet is still in battle, it unfortunately doesn't work. Otherwise you could heal your pet while it is in a battle, which would be unfair for the other opponent... I'd gladly replace your healing potion if you are interested!

It was obviously some weird glitch which didn't recognize me clicking the Game Over button the first time. Thanks for the offer of a healing potion but it was only a 10HP one which I got from a quest. I was going to get the rest of my pet's HP up by fighting Angry Angry Arnold. I'm just glad I was able to sort it out in the end.