Pictures of the woman and her big bird at the airport popped up on Facebook, and someone even recorded her arrival at the airport.
United told Fox News that the woman knew ahead of time the peacock was not allowed on the flight, but she showed up with the bird anyway.
"This animal did not meet guidelines for a number of reasons, including its weight and size," a United spokesperson said.
"We explained this to the customers on three separate occasions before they arrived at the airport,” airline officials said.
There have been complaints in recent months that some people have been violating the federal law that allows service animals on planes by bringing pets and trying to pass them off as therapy animals to avoid cargo fees.