United Airlines is joining Delta in banning emotional support puppies and kittens. The new rules apply to pets under four months old following similar changes by Delta Airlines last month. United will also no longer accept emotional support animals on flights longer than eight hours due to rising complaints. Airlines have been tightening restrictions in an effort to reduce biting incidents and soiled cabins. In recent years, passengers have created a flap by bringing a variety of animals on board including a pig, a turkey and even a duck for emotional support.