Nostalrius Begins - Quality wow vanilla realm (1.12)

The basic pet attack button for my summoned demons is not in my spell book, or my demon's spell book when it is summoned. I accidentally took it off of my voidwalker's action bar when it wasn't locked and now I can't replace it. How do I get this back onto my pet's action bar? Its really annoying me. (Its the attack button with the claw picture) Thanks for any help.