Emotional support animals are animals that "provide companionship and emotional support for people diagnosed with a psychological disorder," according to the AVMA -- and notably "does not have to be trained to perform any particular task."
Some websites have promoted the ability to "Make Your Pet an ESA" in 24 hours.
Another requirement for emotional support and service animals, according to the federal government: The animal must behave properly.
"An animal that engages in disruptive behavior (e.g. barking or snarling, running around, and/or jumping onto other passengers, etc. without being provoked) will not be accepted as a service animal," says the U.S. Department of Transportation's brochure on Air Travel With Service Animals.
Airlines can also exclude animals that are too large or heavy to be accommodated in the cabin or pose a threat to the health or safety of others, according to the DOT.
And, airlines are never required to accept snakes, reptiles, ferrets, rodents, sugar gliders and spiders.
Atlanta-based Delta Air Lines also says it also does not permit hedgehogs, insects, amphibians, goats, non-household birds (farm poultry, waterfowl, game bird & birds of prey), animals "improperly cleaned and/or with a foul odor" and animals with tusks, horns or hooves.
And Delta says it will refuse to transport animals if they engage in “disruptive behavior” such as growling, relieving themselves in the gate area or cabin, or eating off seatback tray tables.
In 2018, Delta tightened restrictions on emotional support animals by requiring a "confirmation of animal training" form and other documents. United Airlines implemented a similar policy.
Delta also last year banned pit bulls as service or support animals, limited each passenger to one emotional support animal, and banned puppies under four months old as service or support animals.
Southwest Airlines, the second-largest carrier in Atlanta, says examples of disruptive behavior that could prompt an animal to be denied boarding include scratching, excessive whining, growling, biting, lunging and urinating or defecating in the cabin or gate area.
Dallas-based Southwest last year limited emotional support animals to one dog or cat per customer and said no other types of animals such as turkeys, pigs or peacocks are allowed.
Other airlines also have their own rules for emotional support animals.
The DOT last year began taking public comments as it considers changing regulations of service animals and emotional support animals on flights.