I can't use my pet Hamtigra in the Battle Coliseum. I have her fully fed and happy. This has been happening for more than a week. I can use my other pets, I don't know what the problem is.

Try going to the Stats page here and see if your pet is currently in a battle. If she's in one you won't be able to start another battle until the old one is finished.

Edit: And pinging got all messed up. Hopefully this fixes it.

Thank you! :D

I feel so silly not realizing >.<

You're welcome! It's a very easy mistake to make and so easy to overlook. Glad it was a simple fix and not actually a bug :)