In Greek mythology, Io was a lover of Zeus, who appeared to her in a cloud. When Zeus's wife Hera suddenly arrived on the scene of the seduction driving her peacock chariot, Zeus turned Io into a beautiful cow to conceal her from Hera. Not fooled for a minute, Hera asked Zeus to give her the cow, and Zeus was forced (by his own weakness) to surrender Io to Hera. The rest is a very entertaining legend for another day.
Jupiter's moon Io
showing the continuously present Prometheus Plume
(near the shadow line at the center of the photo)