Mass. Gen. Laws ch. 129 § 47
A person who transports equine animals for immediate slaughter upon any public way in connection with the purchase or sale thereof, shall have in his possession a bill of sale or memorandum signed by the owner or vendor of such animals, containing the address of such owner or vendor, the date of purchase or sale, the number of animals and means of identification of each animal. Any such person transporting such equine animals shall on demand exhibit such bill of sale or memorandum to any officer qualified to serve a criminal process. No such person shall accept any such equine animal which has been transported over any such way unless accompanied by such bill of sale or memorandum. Such person accepting such animals shall endorse the bill of sale or memorandum in such manner as will signify his acceptance of such animals. Horses intended for immediate slaughter shall not be required to have a certificate as provided in section forty-four.
Mass. Gen. Laws ch. 129, § 47