More and more flights are seeing a wide variety of animals among passengers.

"You get on an aircraft and the cabin looks like a barnyard," Hollis Gillespie, a former flight attendant, told TODAY. "Often it's about the money, because one way to travel with a pet on some airlines can be up to $600, depending on the size of the animal."

>> Read more trending stories

And getting animals onto planes isn't as difficult as one might think. All it takes is an emotional support vest on the animal and an official letter from a mental health professional, both of which are relatively easy to obtain. According to TODAY, there are lots of websites that offer certifications for ESAs. Those seeking the certification can just fill out a questionnaire on one of the sites and an official letter from a mental health professional is emailed without ever even meeting the official.

TODAY's producer Lindsey Bomnin recently filled out the forms to obtain ESA certification for a baby pig. After receiving certification, she attempted to take a trip with the pot-bellied pig. When she showed her documents to TSA, she was cleared to board the flight with the animal in her lap.

"When you try to pass off your animal as a service animal or an emotional support animal and it's not, what you're really doing is you're passing yourself off as a person with a disability, and it can have real consequences for those who actually need these animals," said disability expert Laurel Van Horn with Open Doors Organization.

Plus, untrained animals have the potential to be disruptive during flights, which poses safety concerns.