A Chihuahua in a handbag at 30,000 feet.

A marmoset monkey peering out of his owner's shirt as a Frontier Airlines jet lands in Las Vegas.

A potbellied pig waiting at the gate to board a Delta Air Lines flight in Boston.

Next time you fly, you may encounter an unusual passenger in the next seat. Turkeys, rabbits, roosters, ducks, and geese are legally allowed in the cabins of airplanes as "emotional-support animals." Owners need only a note from a licensed medical professional — which can be bought online — that the companion animal is needed for emotional and psychological well-being.

Airlines are seeing more animals in cabins of planes. Owners can buy medical certificates and service-animal vests online for as little as $40 and avoid airline fees, which can be $75 to $125 each way, for pets to travel.

"The bottom line is it's a bad situation," said Doug Lavin, vice president of the International Air Transport Association (IATA), which represents 269 airlines around the world. "The numbers are quite high — in a six-month period from November 2015 to April 2016, one major carrier had 82,000 service animals, of which 54,694 were emotional-support animals."

"There is a good amount of fraud," Lavin said. "If you go online, you can find sites and order, for a small fee, a letter from a licensed professional that says you need to bring your potbellied pig on the plane."

The growing menagerie — only snakes, other reptiles, rodents, ferrets, and spiders are banned — is hurting travelers with a legitimate need for service animals, Lavin said. "A plane can only handle so many animals."

The Department of Transportation last year convened airlines, flight attendants, aircraft manufacturers, and disability rights groups to recommend new standards for animals in passenger cabins. But the committee could not agree. "We're kind of back to square one," said Paul Hudson, president of FlyersRights.org, a consumer group representing airline passengers.

"We think there does need to be regulation, and there is a lot of abuse going on," he said. "We have had complaints from our members who are allergic to animals, to dogs or cats."

The Americans With Disabilities Act defines service animals as dogs, and, in some cases miniature horses, that are trained to perform tasks for people with disabilities. The ADA does not recognize emotional-support animals as service animals.

However, the Air Carrier Access Act that governs airlines is different and includes emotional-support animals as service animals.

Both fly for free and are allowed uncrated on the plane, in their owner's lap or at their feet. Pets, on the other hand, must fit in carriers under an airline seat, or in the cargo hold.

Airlines proposed restricting service animals to dogs, capuchin monkeys, and miniature horses, and emotional-support animals to dogs only. Disability groups do not want to severely limit the species.

"While there may be changes that could be made to the current regulations," emotional-support animals play "an important role" for the people they assist, said Heather Ansley, associate general counsel at Paralyzed Veterans of America.

Carla Fitzgerald of Milwaukee has an emotional-support animal, a duck named Daniel. Fitzgerald suffers from post-traumatic stress disorder after a 2013 accident.

Her treatment includes seeing a clinical psychologist weekly and a pain doctor at least once a month, and she is scheduled to begin a spinal-cord "stimulator" trial in March in hopes of mitigating pain signals from her brain.

Fitzgerald's psychologist wrote a letter endorsing Daniel, who goes everywhere with his owner, including a flight in October from Milwaukee to Charlotte, N.C., and to Asheville, N.C. It was Fitzgerald's first flight since the accident.

"When I'm in lots and lots of horrific pain, Daniel will spend the whole day at my side. He is able to stop and redirect a PSTD episode so it doesn't get bad for me," Fitzgerald said. "He will give me a cue to lie down. He'll get on my chest and blow kisses."

Fitzgerald agrees there should be restrictions for animals on planes, including "some kind of waste-management system." Daniel wears a diaper. "Also animals shouldn't bite or be loud. You don't want to sit next to an animal and have to worry, Is this animal going to bite me, or peck me, or crap on the floor? That's not cool."

Taylor Garland, spokeswoman for the Association of Flight Attendants, said: "Service animals play a vital role in the lives of people with disabilities." But when animals are not properly trained, run around the plane, bark, or attack people or other animals, "it creates an unsecure cabin."

At times, passengers are startled by who shows up for their flight.

Romila Mushtaq, an Orlando physician, was at Boston Logan Airport in January 2016 when she noticed waiting at the gate to board was a pig. "Everybody thought it was cute until you realized: Is this uncaged pig going to be sitting in the seat next to me?"

Mushtaq said that as a medical doctor she was "very familiar" with specially trained guide dogs for the visually impaired and dogs used with diabetic patients to detect hypoglycemia and with epilepsy patients to detect seizures.

"I thought, oh, is this some other trend with animals that I'm not aware of? I did the research and could find no evidence of pigs providing emotional comfort overall. There is no literature. It just made me wonder — how did this woman get the official leash or paperwork to get a pig on the plane?"

"As a frequent flier, I find it concerning," she said. "At what cost does somebody have an emotional-support animal because they want comfort on a flight, and at what cost to the discomfort of the airline staff and the other passengers? I would have been irate had I been seated next to the pig."