...I can't seem to get my pet into the coliseum? I'm fairly new to battling, but I've gone through plenty of fights already. She's at 10/10 hunger and happiness, full HP, not in any fights already underway... but the challenger page tells me I don't have any eligible pets.

Is there something I'm doing wrong here?

Was your pet recently transferred to you from another user? If so, transferred pets cannot battle for 30 days.

No, I made her myself and I've had her for a couple months. And she's not in the training center either. Is it because I recently spent experience points on her?

I also had this issue a few months back. What happened was that Italia (my battle pet) was still in a battle without me knowing it. This might be the case with your pet, so try checking if she's still in a battle by clicking on the statistics page of the BC. If she is, you can either finish the battle or forfeit it. After that, you should be able to start battling again if this is the problem.

Oh my god you're right. It was a battle I thought I had already completed, and the status was that the battle was over, waiting for me to end it.

I was right. It was a dumb mistake. Thank you all for your patience though, I'll be sure to be more wary of that next time.

Glad I could help! Don't stress about it, everyone has these little issues sometimes. :)