This was a pig that truly could not fly.

The pig was ordered off a U.S. Airways plane at Bradley International Airport in Connecticut on Wednesday after crew members determined the animal had become disruptive, said Laura Masvidal, a spokeswoman for U.S. Airways parent American Airlines.

The pig had been brought aboard the flight by a passenger as an emotional support animal, Masvidal said. She said both the pig and its owner left the aircraft before it took off. She had no other details and did not immediately know where the flight was headed.

Jonathan Skolnik, a University of Massachusetts professor who was on the flight, told ABC News that he initially thought the female passenger was carrying a duffel bag.

"But it turns out it wasn't a duffel bag. We could smell it and it was a pig on a leash," he said.

The woman sat in an empty seat next to him and tethered the pig — which he estimated weighed 50-70 pounds — to the arm rest, but the animal began walking back and forth, Skolnik said.

Emotional support animals are allowed on commercial flights under U.S. Department of Transportation rules as long as they are not disruptive, Masdival said.