Airlines, meanwhile, are trying to stem the stampede of animals in the cabin. Starting Thursday, Delta Air Lines and United Airlines will require passengers who wish to bring an emotional support dog, duck or other animal to bring signed documents that state their animal can behave in the cabin.
Passengers and airline employees have complained of soiled cabins, allergies and even biting by some of the animals on board. United recently turned away a peacock one passenger wanted to bring with her aboard a cross-country flight as an emotional support animal.
Emotional support animals are allowed in cabins free of charge under the 1986 Air Carrier Access Act, which stands in the way of airlines barring the animals altogether.
Animals in airport programs face a higher bar than some animals passengers bring on board.
"There's a whole onboarding process," said Heath Montgomery, spokesman for Denver's airport, adding that the goal is to add more animals to the CATS program. The program's administrators aim to bring the fuzzy volunteers past security, near gates, where passenger stress can run high as they face delays and lengthy, and sometimes confusing, boarding processes.
Volunteer animals go through an interview process to see how they interact with people in the airport and have to present documents showing they've undergone therapy training.
The airport's animals are easier to handle. They're not in a confined space, like they are in an airplane cabin, Montgomery notes.
"It's a completely different type of environment," Montgomery said.