Last week, a monkey was spotted in the cabin of a Frontier Airlines flight from Columbus, Ohio, to Las Vegas. Which had us wondering …

1. Why was the monkey on a plane? It was a passenger’s emotional support animal. Individual airlines determine whether to allow ESAs (most do), so passengers should check with their carrier before booking a flight with an animal.

2. What is an emotional support animal? An assistive aid prescribed by a doctor for a patient’s mental health. “Just the presence of [an emotional support animal] helps lower somebody’s anxiety level,” says Stephanie Gerken, founder of local nonprofit Michael's Angel Paws.

Unlike service animals, ESAs are not trained to perform tasks. “Service animals are there to do a particular task to help mitigate somebody’s disability, whether visible or invisible. Service animals require intensive training, usually 18-24 months,” Gerken says.

3. What animals can’t fly? Frontier Airlines spokesman Richard Oliver says, “All types of service animals are allowed in the cabin, including service animals in training,” on Frontier flights. “Uncommonly used animals like miniature horses and monkeys can travel as service animals, although a particular animal may be denied transportation on a case-by-case basis if it is too large or heavy to be accommodated on a flight.”

Species not accepted in the cabin include: mice, rats, squirrels, beavers, ferrets, reptiles (including snakes), amphibians, spiders and insects. A service animal can be out of its container as long as it’s harnessed or leashed, but it “needs to remain under control of the traveler or traveling companion,” Oliver says. “The animal can sit on you or with you at your feet, but it cannot obstruct the aisle or the seat next to you.”