It's not uncommon these days to see small dogs or cats in carry-on kennels stowed under the seat in an airplane passenger cabin. But more and more, the diversity of animals on U.S. airplanes has extended to creatures such as birds, pot-bellied pigs, and even a kangaroo or two taking a seat to themselves. Most of these pets are categorized as emotional-support animals, a kind of service animal certified by a mental-health professional that helps provide comfort to its human companion and minimize the symptoms of an emotional or psychological disability. Emotional-support animals can provide very real help, but there is controversy over how easily a passenger can get their pet certified. Since large pets must fly in the baggage compartment for a hefty fee and, thanks to the U.S. Air Carrier Access Act, emotional support animals fly free in the passenger cabin, there's incentive for those without a disability to get their pets certified. This has caused passenger complaints about service animals to rise dramatically in the last few years.