The Hen in a Hat from Dr. Seuss's ABC is revealed to be Harriet in the Living Books version.
She has a thing for square dancing. Sometimes in her big hat she can get hot and sweaty most of times so Icabod and Izzy get the water hose and the hydrant and doses her with water to cool her off. She later discovers a hidden Hornet flying in her hat. She enjoys the music that Icabod and Izzy play from a honking horn harp.
Sometime later, she leaves her hat behind walking off after Icabod and Izzy leave.