According to The Sun, Ronia Froese took the horse with her into the cabin as an emotional support animal, flying from Michigan to California.
Fred the horse was dressed up in a Mexican wrestler suit and had been specially trained through Little Horses Big Smiles, a non-profit animal therapy training centre, to make the trip.
"I paid an arm and a leg for tickets," she said.
"But I did so because it was Fred's first time and I wanted him to be comfortable. I wanted him to have the most room."
She said that apart from the initial confusion and a few strange looks, everyone on board was "sweet as pie".
Although many US airlines allow support animals onto their flights, the US Transportation Department recently announced plans to tighten regulations so that only trained dogs are allowed on flights.