Travel TV show "The Jet Set" posted photos of the bird atop a luggage cart on its Facebook page.

"This animal did not meet guidelines for a number of reasons, including its weight and size," United spokeswoman Andrea Hiller told CNBC. "We explained this to the customer on three separate occasions before they arrived at the airport."

United Airlines said a woman brought a peacock with her for a flight Saturday from Newark Liberty International Airport, despite several warnings that the bird did not satisfy the airline's guidelines. The bird did not fly.

Airlines are cracking down on emotional support animals, starting with peacocks.

The bird appeared to be a peacock named Dexter, according to its Instagram account, that included another photo.

The incident occurred as airlines are rethinking their policies toward support and service animals, after carriers and passengers have faced soiled cabins, bitten passengers, complaints about allergies and other issues.

Earlier this month, Delta Air Lines announced that starting March 1 it would require passengers who wish to bring a support animal on board to bring a signed document stating the animal is trained, an effort to make sure aggressive pets don't make it — cage-free — into the cabin.

Under the 1986 Air Carrier Access Act, support and service animals are allowed into the cabin free of charge.

But there's a gray area. The Department of Transportation said airlines are "required to accommodate passengers with disabilities who depend on the assistance of service animals within limits."

The airlines are "not required to accommodate unusual service animals such as snakes, reptiles, ferrets, rodents, and spiders," it added.

Airlines are also allowed to refuse to carry animals due to their weight or size or if the animal poses a safety or health threat and even if it would disrupt cabin service.

Now we know a peacock fits that bill.