So I hatched a new pet yesterday because of double xp and I wanted to start training it early so I decided to hatch it early with gold...mistake. After I hatched it it was still greyed out and I can't equip it. It doesn't show the egg anymore, it shows the pet, but it's acting like it never hatched. I tried letting it sit overnight to see if it would fix the problem and it didn't, so now I'm down like 120k gold and a good pet that was the one I wanted.
Is it a school-only pet? For example, a Life Dryad pet can only be equipped by a Life wizard. If that's the problem, try transferring the pet to a different wizard on your account.
If that's not the problem, maybe try contacting support. I know that won't help you with the lost gold and lost double xp opportunity, but recovering use of the pet might help you somewhat.
Alia Misthaven