I wanted to challenge an opponent in the Coliseum but it seems that I cannot. However, the pet I want to use is happy and full. Her HP is full as well.

Is it a glitch?

If you file a Ticket a User Admin can most likely unstick your pet for you. Alternatively, you may also be able to unstick it by going to "Battle Coliseum", then "Statistics".

Your pet you're trying to fight with might be in a battle already? Like you never hit game over on a fight you were in before? shrug That's happened to me before.

Oh, that's what was wrong. Woopsies!