The number of pets on planes has become a hot-button issue of late as emotional support animals have become more common than ever. ( SHELLY YANG/KANSAS CITY STAR / TRIBUNE NEWS SERVICE VIA GETTY IMAGES)

The U.S. Transportation Department is considering a change that would make the only service animal allowed on planes "a dog that is individually trained to do work ... for the benefit of a person with a disability" and allowing airline employees to request forms attesting to the dog’s training. This comes in response to customer and airline complaints about service and emotional support animals’ disruptive behavior.