Terrapin

Question
My terrapin has an air bubble on the side of his head, what can this be?

Answer
At a guess, it could be any number of things, but if it's located behind the eye it's most likely an ear abscess.  You need to take him to a herp (reptile) vet for treatment, which will probably include lancing the abscess and a course of antibiotics.  Please don't attempt to lance the abscess yourself, as this could be very dangerous.  This is not something that can be treated at home!  Good luck, and I hope your terrapin recovers completely.