Flying with your emotional support peacock or hamster? No. But with your service dog, or even two -- that's okay on US airlines, regulators have decided.

The US Department of Transportation issued a final rule Wednesday on what kind of "service animals" that airlines are required to accommodate on board flights in the United States. 

The practice of flying with a pet for emotional support has exploded over recent years, with travelers seeking to board with all sorts of non-human mental health aides: pigs, parrots, monkeys, ducks, a peacock, and even, in one genuine attempt, a miniature horse.

That has challenged airlines pressured by mental health advocates to accomodate such travelers.

But, the department said, there has been a rise in disruptions on board by "unusual species" and misbehaving animals, "which has eroded the public trust in legitimate service animals."

The Transportation Department's conclusion? Only man's best friend, canines, qualify as "service animals" for special boarding permissions to accompany people with physical, psychiatric or mental disabilities.

The rest are just "pets."

The ruling defines a service animal as "a dog that is individually trained to do work or perform tasks for the benefit of a person with a disability."

That means that airlines will require Transportation Department forms attesting to the dog's health and training, and for long flights, that the dog "can either not relieve itself, or can relieve itself in a sanitary manner."

That said, a person who qualifies can bring up to two service dogs with them, and the airlines can not discriminate on breed. Some had sought to exclude animals from breeds known to be more aggressive.

As for other animals, even those whose owners have a doctor's statement that they are necessary for "emotional support"?

They don't qualify as service animals, at least for airline regulations, the department said.

Never fear: travelers can still bring their small pets aboard, in cages to tuck under their seats, at the airline's discretion, often paying an extra fee.