Its the theobromine in cocoa that is toxic to the dogs, it is quite definitely not a myth. I'm training as a vet and the amount of dogs i've seen come in with theobromine poisoning is insane because the owners all thought it was a myth. As Gr3tta rightly said, the darker the chocolate the higher the amount of theobromine and the worse the poisoning. Despite small amounts not causing too much harm its never a good idea to give your dog chocolate, it might not give out any obvious signs but it can give them one hell of a stomach ache which is none too pleasant for them.