AN airline rule change will allow miniature horses on board if they are classed as service animals.

Southwest Airlines announced those trained to support individuals with a disability will be allowed to fly.

The company say "service animals must be trained to behave in a public setting and must be under the control of the handler at all times".

The U.S. airline also announced it would limit emotional support animals to only dogs and cats on its flights and will allow one per passenger.

According to guidance from the Americans with Disabilities Act, they must be under 86 centimetres tall and 45 kilograms heavy.

Southwest updated its service animal policies and added that the emotional support animal must remain in a carrier or be on a leash at all times.

They said that customers traveling with pets will need to present a complete letter from a medical doctor or licensed mental health professional on the day of the departure.

Airlines have started to tighten policies to fly animals on growing concerns about passengers bringing aboard exotic pets that could pose a safety risk.

There are also growing complaints of some passengers falsely claiming pets as emotional support or psychiatric service animals to avoid paying.

The U.S. Transportation Department told U.S. airlines in may that they must continue to allow the transport of the most common service animals, but said it was asking for public comment about amending its existing regulations.

Southwest said it would formally accept fully-trained psychiatric support animals as trained service animals.

The changes would be come into effect on Sept. 17, it added.

Rival Delta Air Lines Inc tightened its requirements for passengers traveling with onboard service and emotional support animals earlier this year.

Alaska Air Group Inc has also introduced new rules.