The airline said in a statement that the unidentified woman made note of the fact she was bringing an emotional support animal with her, but did not specify the nature of the animal.
Once Frontier employees realized she'd brought a squirrel on the flight departing from Orlando, they asked the woman to leave the plane.
"Rodents, including squirrels, are not allowed on Frontier flights," the airline said in its statement. "The passenger was advised of the policy and asked to deplane."
However, according to the airline, the woman refused to leave her seat, so police were called to remove her.
Video recorded by a passenger showed people celebrating as she was removed.