You don't need anything to enter Mexico. To return to the US, all you need is proof of rabies vaccination, and only for dogs (not cats).
The official rules:
"A general certificate of health is not required by CDC for entry of pet dogs into the United States..."
"Dogs must have a certificate showing they have been vaccinated against rabies at least 30 days prior to entry into the United States." CDC Rules
A unofficial site says:
"A valid rabies vaccination certificate should accompany the animal. This certificate should be in English or be accompanied by a translation. It should identify the animal, the dates of vaccination and expiration, and be signed by a licensed veterinarian." Importing dogs
Take that for what it's worth.
Dave from AK
2004 Beaver Monterey 40'
2000 Ford Explorer