The old expression, "when pigs fly," is close to comming true.

Although items like snow globes are still not permitted aboard planes, travelers in Britain might soon find that they're sharing airplane seats with more exotic passengers, like pot-bellied pigs, monkeys, and miniature horses. These animals, authorities say, help provide "emotional support" for their human owners, according to an Australian news website.

New guidelines for the country's Department of Transportation are part of a draft manual on equality for disabled people travelling on commercial passenger planes, reports the Daily Mail.

It's not a free-for-all, however. To avoid aircrafts becoming an airborn zoo, transportation officers must determine if the animal is permitted on the plane by running through a list of guidelines. First, to figure out if these animals really do perform a service to the passenger, employees are encouraged to ask about the kind of training the companion has received and how the human counterpart is helped by the animal. For instance, miniature horses are helpful for visually-impaired passengers, while pigs are more often used for people who need assistance but are allergic to dogs.

Once the employee decides that the animal is, in fact, a service animal, the manual dictates that they "must permit the service animal to accompany the passenger to her seat provided the animal does not obstruct the aisle or present any safety issues and the animal is behaving appropriately in a public setting."

The owner has to take responsibility for the animal and must also provide a ''relief area'' for the companion.