Many have heard of "Snakes on a Plane," but how about a pig on a plane? A U.S. Airways flight out of Connecticut on Wednesday took off without two originally planned passengers: a woman and her "emotional support animal," which just happened to be a pig.

Jonathan Skolnik, a professor at the University of Massachusetts Amherst, told ABC News on Thursday that he was on the flight Wednesday when a woman with a duffel bag sat next to him. "But it turns out it wasn't a duffel bag," he said. "We could smell it and it was a pig on a leash. She tethered it to the arm rest next to me and started to deal with her stuff, but the pig was walking back and forth."

Guessing that the pig weighed up to 70 pounds, Skolnik said, "I was terrified, because I was thinking I'm gonna be on the plane with the pig."

American Airlines, which owns U.S. Airways, told ABC News that the pig, which had been brought onto the plane as an emotional support animal, had become "disruptive," and as a result, she and the pig were asked to leave the flight.

It used to be the case that seeing an animal in a restaurant, theater or on a plane was rare, and if it was allowed, the animal was a dog that helped blind or deaf people. In 2011, the Department of Justice, which oversees the Americans with Disabilities Act, excluded emotional support dogs, and defined service animals as dogs that are “individually trained to do work or perform tasks for a person with a disability,”according to the American Bar Association Journal. But the Air Carrier Access Act, which regulates commercial air travel, allows for service animals who provide emotional or psychiatric support -- which has caused a rise in turtles, cats, and, yes, pigs -- on planes, as long as the passenger provides a note from a mental health professional.

In addition to freaking out passengers like Snolnik, who probably didn't anticipate sitting next to a pig when he bought his plane ticket, disability and service animal advocates worry that legitimately disabled people could be hurt by the expanded allowances, particularly if there are people faking the need for emotional support animals. 

Regardless of the long-term repercussions of Wednesday's booted-off pig, it looks like the folks at Passenger Shaming, which describes itself as "Photos of a--holes taken by anonymous flight attendants and passengers from all over the world” and posted to Instagram, Twitter and Facebook, is going to have more material for its posts.