What are the criterias we should follow in the frontier ? In terms of frontier pet policy : On all of their domestic flights, Frontier accepts a wide range of small animals, including domesticated dogs, cats, small birds, hamsters, rabbits, and guinea pigs. Only domesticated dogs and cats are allowed on international flights. ESAs are no longer eligible for free flights on Frontier, and they must follow the same rules as other pets. You must complete the three forms listed on Frontier's website in order to be eligible. A medical/mental health professional form, a veterinarian health form, and a passenger acknowledgement form are all included in the documentation.