All the passenger has to do is say the animal is a service animal and they don't have to show anything to bring the animal on-board, without cost, and outside a carrier. It is far, far easier than claiming the animal is an emotional support animal.

The only thing the airline can do is ask the passenger what type of service the animal provides (they can't ask what health issue the passenger has) and as long as the answer is credible, the animal goes on board.

bdemaria, the ADA does not apply to airlines. It is the ACA, i.e., the Air Carrier Access Act.