Like Yookoohoos generally, Mrs. Yoop is a cryptic figure about whom little is known. She lived or lives in her castle in a remote area of the Gillikin Country; her usual form (not necessarily her natural form) was that of an attractive but gigantic woman, several times taller than a man. She was or is the wife of the giant Yoop who is imprisoned for his acts of cannibalism. (The Tin Woodman of Oz)
HistoryShe enjoyed transforming unlucky travelers who stumbled into her clutches into comical and ironic forms. Most famously, she transformed the rainbow fairy Polychrome into a canary, the boy Woot the Wanderer into a green monkey, the Tin Woodman into a tin owl, and the Scarecrow into a straw-stuffed little brown bear.
Yookoohoo magic is so strong that Ozma had great difficulty restoring Woot to his proper identity. In the end she was able to do so only by turning Mrs. Yoop into the green-monkey shape, which deprived her of her magical abilities. The change is permanent. Once Mrs. Yoop was deprived of her magic, she could be safely ignored. ("The Tin Woodman of Oz")
Paul Dana provides an origin for Mrs. Yoop in his novel Time Travelers of Oz. And she returns, still in her green monkey form, in John W. Kennedy's A Grown-Up in Oz and in Dennis Anfuso's A Promise Kept in Oz.