Here's how airlines decide if a pet qualifies as an emotional support animal (DAL, UAL, AAL)

Airlines are worried about passengers bringing fake emotional support animals onto their flights.
Delta, American, and United require passengers to provide a signed document from a mental health professional to verify that an emotional support animal is not merely a pet.
There are some animals that airlines do not have to accept as emotional support animals, including reptiles, snakes, and spiders.

Airlines are worried about passengers trying to pass off pets as emotional support animals, which fly for free. But airlines are not always clear about what qualifies as an emotional support animal.

Sometimes, it's obvious, like the recent incident involving a woman who unsuccessfully tried to designate a peacock as an emotional support animal. But in other cases, it might not be as clear where to draw the line between pet and medical necessity.

After Delta Air Lines announced new regulations that require passengers to provide documentation verifying their emotional support animals have been trained and vaccinated, we looked at the rules Delta, United Airlines, and American Airlines set out for emotional support animals. We found that while they require passengers to provide documentation proving their emotional support animals are not merely pets, they leave the job of determining what qualifies as an emotional support animal to licensed medical professionals — with some exceptions.

Here are the steps passengers have to take to bring an emotional support animal into the main cabin on one of the three major US airlines:

American — Passengers must submit a document signed by a licensed doctor or medical health professional which states that the passenger has a "mental health or emotional disability" and needs the animal "for emotional support or psychiatric service" on the flight or at the passenger's destination. The document needs to have been signed within the past year and must be submitted at least 48 hours before the flight.
Delta — Starting March 1, passengers will have to submit three documents if they wish to travel with an emotional support animal. In addition to a signed statement from a medical professional, passengers will have to provide vaccination dates and the contact information of the animal's veterinarian and sign a statement that claims the animal is properly trained "to behave in a public setting" and take the passenger's "direction upon command." The document needs to have been signed within the past year and must be submitted at least 48 hours before the flight.
United — Passengers must submit a document from a medical or mental health professional which states that the passenger has a "mental health-related disability" and that the emotional support animal "is necessary to the passenger's mental health or treatment." The document needs to have been signed within the past year and must be submitted at least 48 hours before the flight.

While passengers who can complete these steps will often be able to bring their emotional support animals into the cabin with them during a flight, there are some exceptions.

The US Department of Transportation says that airlines don't have to let support animals into the cabin if they're too large, heavy, disruptive, or threatening to the crew and passengers. And airlines don't have to accept ferrets, reptiles, snakes, spiders, or sugar gliders as emotional support animals. American Airlines adds hedgehogs, insects, rodents, amphibians, non-household birds, and any animals with tusks, horns, or hooves, to its no-fly list.

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