You've heard the phrase "when pigs fly." Well, it's happening. Pigs, and even turkeys, have been boarding airplanes as emotional support animals. As Time Warner Cable News reporter Alberto Pimienta shares, it's triggering a debate at the federal level.

Many people were shocked recently when they saw a pig on board an airplane - and a turkey - but the unexpected passengers are considered emotional support animals.

The pets can help their owners cope with mental illness.

"For some people, an animal is like their child and what - you’re going to make them take medication to get on a plane?" said Autumn Seibel, pet owner.

Owners can fly with their emotional support animals if they have the proper documents. But this is causing problems. The Department of Transportation created a committee to bring together airlines and advocates to try to limit the definition of emotional support animals.

Alicia Smith is on the committee. She travels with a service dog and says the value of service and support animals can't be argued.

“We absolutely want them to continue," Smith said. "They have been around for quite a while. In our opinion the airlines really didn’t have compelling information to disprove their value.”

The National Alliance on Mental Illness agrees there should be species limitations. Airlines say some pet owners are getting questionable documentation from websites so they don’t have to pay fees to bring their animals on board.

“As advocates, we call them certificate mills. We’d like them to be out of business," said Smith.

“It is literally becoming a dog and pony show onboard," said the Association of Professional Flight Attendants in a statement. "In some cases the passengers’ comfort animals are making the passengers around them uncomfortable.”

The negotiations in the committee went on for months, but the airlines and the advocates couldn’t agree - so the final decision will come straight from the Department of Transportation. It could be years before the new rules are implemented.