My battle pet is bloated and happy but I get the "Oh no! None of your pets are eligible for battle. You might want to make sure that they're well fed and happy!" message every time I visit the battle coliseum.

Am I missing something?

Are you already in a battle? Go into the statistic link and you'll see.

Ah, you're right. I had no idea he was in battle. Thanks for helping! :)

You're welcome!