Bookrane

This swane always looks threatening and makes terrifying hissing sounds, but it's actually a real coward that hides as soon as danger is near. However, it can run very long distances without ever slowing down or complaining, no matter how much weight it bravely carries on its back. So, it's only natural for it to go with the Master every time they set off on an adventure. (Place this petsmount over another, already-equipped petsmount to change its appearance.)