Pigasus is a flying, winged pig whose riders are magically compelled to speak only in verse until they dismount.
At some point he came into the possession of seamen riding in a seagoing castle. There, he was discovered by Peter Brown, when Samuel Salt's crew boarded the castle and ransacked it. They traded a Banany Goat for Pigasus. (Pirates in Oz)
When a false king replaced Ozma, Dorothy Gale was the only one who maintained her memory of the true queen. She mounted Pigasus, who immediately gained a true memory since he could see her thoughts as long as she was riding him. The two flew away to find help to restore Ozma. Through many adventures they succeeded. (The Wishing Horse of Oz)
Besides Ruth Plumly Thompson, others (non-Oz writers), like John Steinbeck, have made use of the image of a flying pig called Pigasus.