玉兎
The dark spots visible on the full moon are said to resemble a rabbit who lives in the moon.
In Japan, the rabbit is described holding a wooden mallet which he uses to pound mochi (rice cakes) in an usu, or mortar. The mallet and mortar as also visible as dark spots on the moon. In China, the rabbit is believed not to be creating mochi, but is instead mixing the medicine of eternal youth.
The myth of the rabbit in the moon is very ancient. The earliest written version comes from the Jātaka tales, a 4th century BCE collection of Buddhist legends written in Sanskrit. The legend was brought along with Buddhism from India to China, where it was blended with local folklore. It came to Japan in the 7th century CE from China, where it was again adapted and adjusted to fit local folklore.
The Japanese word for pounding mochi in a mortar like the rabbit is doing—餅搗き (mochitsuki)—and the word for the full moon—望月 (mochitsuki)—are homophones.