Yes, the Winged Monkeys are people in costumes in "The Wizard of Oz."
Specifically, only one of the Winged Monkeys is credited in the official cast list of the beloved 1939 film version of "The Wizard of Oz." The part is listed as that of Nikko. The actor listed as playing the head of the Winged Monkeys is Pat Walshe (July 26, 1900 - December 11, 1991).