Along with that spike, there's been an increase in onboard incidents involving those animals.
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The last straw, when a passenger arrived at Newark airport with an emotional support peacock.
WATCH: Peacock denied as emotional support animal for flight
The video went viral and started a bigger discussion.
A new United Airlines policy kicks in today.
It's important to note this only applies to emotional support animals, not trained service animals.
Passengers with emotional support animals will have to provide the following:
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A vet health and vaccination form signed by the animal's veterinarian.
A letter from a mental health professional.
A signed form that their emotional support animal is trained to behave properly in public settings.
That form also confirms that a passenger acknowledges responsibility for the animal's behavior.
All three documents must be provided to United Airlines 48 hours in advance of the flight.
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Delta Airlines is also rolling out new rules for emotional support animals. They require the same type of documents.
MORE: Delta to crack down on service, emotional support animals
This is a move that's been praised by the Association of Flight Attendants.
Those passengers who don't provide these documents will have to pay for their animal's airline ticket, which in some cases costs hundreds of dollars.
RELATED: Texas State student: Airline told me to flush pet hamster down toilet
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