Yookoohoos are a powerful and dangerous type of witch who specialize in transformations.
Since Yookoohoos are extremely rare, and only two are known to exist, there is little that can be said about them with certainty. Both known Yookoohoos are females who live in remote locations, and tend to be self-absorbed, and indifferent or hostile to strangers. Intruding upon a Yookoohoo is a very risky thing to do. (The Tin Woodman of Oz)
Yookoohoos like to keep curious pets that they can shift into exotic shapes.
Given that Yookoohoos can transform themselves as well as others, it is difficult to determine their natural forms. One Yookoohoo apparently prefers non-human forms over human shape for normal existence. (Glinda of Oz)
A Yookoohoo can be human-sized, or a giant several times human size. Mrs. Yoop, the giantess Yookoohoo, has an estranged husband who is also a giant (21 feet tall), but apparently not a Yookoohoo: the Yoop is imprisoned due to his bad habit of eating people, and is unable to escape his confinement through transformation. (The Patchwork Girl of Oz)
The Yookoohoo Reera the Red is less aggressive than her giantess counterpart. She is not friendly to humans, but is not overtly hostile, and can even be lured or manipulated into helpful action. (Glinda of Oz)
Both known Yookoohoos use physical aids in their magic. Mrs. Yoop has a magic apron; Red Reera employs a mysterious substance in her transformations. Both Yookoohoos lived in the Gillikin Country; whether they are native to that region, or whether they have settled there because of its relative isolation and wildness, is uncertain.