The U.S. Department of Transportation could allow airlines to ban untrained animals from airplanes, possibly ending the debate over whether emotional support pets deserve the same treatment as fully trained service animals.

The agency proposed new rules Wednesday after years of controversy over what kinds of animals should be allowed on airplanes and what qualifies as a support animal.

Airlines and passengers have been grappling with letting animals fly in the cabin on airplanes, particularly in recent years with a rise in emotional support animals ranging from dogs and snakes to miniature horses, pigs, ducks, peacocks and turkeys. But some passengers worry that animal owners are skirting rules for the convenience of flying with their pets as airlines tried to accommodate federal rules for people with disabilities.

The new rule would let airlines limit service animals to only dogs and prohibit those “emotional support animals” that require little or no documentation. There are online companies that will sell certification for emotional support animals with a therapist’s signature for less than $100.

The DOT’s proposed amendment could be in effect later this year after a 60-day public comment period. It would allow for psychiatric service animals, but only after they’ve had “the same training and treatment of psychiatric service animals as other service animals."

The rights of pet owners and other passengers have sometimes come into conflict, such as when a Texas man sued American Airlines after he said he was removed from a plane in favor of an emotional support animal he was allergic to. Delta Air Lines has come under fire for banning certain breeds of dogs, such as pit bulls and terriers.

“The days of Noah’s Ark in the air are hopefully coming to an end,” said a statement from Association of Flight Attendants-CWA President Sara Nelson. “Passengers claiming pets as emotional support animals has threatened the safety and health of passengers and crews in recent years while this practice skyrocketed. Untrained pets should never roam free in the aircraft cabin.”

Under the proposed rule, airlines would be allowed to require a form for service animals attesting to good behavior, health and the ability for the animal to relieve itself in a sanitary manner. It would also limit the number of service animals per passenger to two and let airlines require service animals to be in a harness or leash. Airlines also could require service animals to fit in the space around a passenger’s feet.

The federal government is seeking comments on the new rules. To comment, follow this link.