The Department of Transportation (DOT) is seeking public comment on a proposal issued Wednesday defining what can and cannot be labeled a service animal on an aircraft.

The DOT explicitly defined a service animal as as “a dog that is individually trained to do work or perform tasks for the benefit of a qualified individual with a disability, including a physical, sensory, psychiatric, intellectual, or other mental disability.”

Because of this definition, airlines have greater choice in whether to allow a dog to stay in the cabin with its owner. The DOT said the definition was restricted to dogs because other animals, such as miniature horses, “are not flexible” on an aircraft.

According to the DOT, passengers with a disability would be limited to two service animals per passenger and all animals would have to be leashed or harnessed.

The agency also put additional limits on emotional support animals, saying that while these animals can be recognized as service animals, “airlines do not have to recognize them.”

Owners with disabilities would also have to fill out three universal DOT forms before their animals would be allowed to board an aircraft.

One form would have to show their animals’ history of training and good behavior, another would have to provide proof that the animal is in good health, and the final one would have to demonstrate that the animal has the ability to “not relieve itself” or “relieve itself in a sanitary manner” on flights more than eight hours long.

The proposal is still in its introductory stage and has not yet reached the final rule-making period.

Airlines have already taken steps to limit their emotional support animals on its flights.

In December 2018, Delta Airlines announced it would ban all emotional support animals from flights over eight hours and ban all service and emotional support animals under four months old on all airplane routes.

Southwest Airlines announced in August 2018 that emotional support animals are limited to dogs and cats, and all animals must be accompanied with a doctor’s note.