In Wonder Woman #128 (Dec 1961), the writers struggle to figure out how to solidify the “Wonder Family” by having Wonder Woman tell the origin story of her invisible robot plane to recordings of herself at younger ages as Wonder Tot and Wonder Girl.
As Wonder Woman is about to leave Paradise Islands, she realizes that her powers of “riding air currents” will not be sufficient to deal with criminals in planes or space invaders. She asks Athena, who guides her to seek out an “aerial steed” at the bottom of the ocean. There, she meets Manno, the Mer-Man, and together they encounter the Pegasus, a winged horse.
After taming the winged stallion, Wonder Woman and Pegasus battle with a prehistoric winged dinosaur to fill up a few panels, and then – for no apparent reason – Pegasus transforms into a robot plane. ¯\_(ツ)_/¯
