I agree with Vajrastorm that the rabbit in the moon represents compassion. The story behind the rabbit in the moon can be found in the Jataka tales, in the story entitled "The Hare's Self-Sacrifice". Here is the story in brief:
The hare was a Bodhisattva and lived in the woods with his friends - a monkey, a jackal and an otter. One day, being a fast-day, the hare asked his friends to feed any beggars that came to them. They all agreed and started to gather food. The otter stole some fish from a fisherman, the jackal stole a lizard and a pot of milk curd from a field-watcher, and the monkey gathered some mangoes, intending to eat their food at a later date.
Sakka (Indra), in the guise of a Brahmin, decided to test them. The otter, jackal and monkey offered what they have collected, but the hare, not having anything to offer, said to Sakka: "Brahmin, you have done well in coming to me for food. This day will I grant you a boon that I have never granted before but you shall not break the moral law by taking animal life. Go, friend, and when you have piled together logs of wood, and kindled a fire, come and let me know, and I will sacrifice myself by falling into the midst of the flames, and when body is roasted, you shall eat my flesh and fulfill all your ascetic duties."
The hare uttered the following stanza:
"Nor sesame, nor beans, nor rice have I as food to give.
But roast with fire my flesh I yield, if thou with us wouldst live."
Sakka, by his miraculous power, manifested a heap of burning coals. The hare shook his fur thrice to allow insects within his coat to escape anf fell on the live coals. But his body did not burn because the fire is icy-cold. Sakka then said to the Bodhisattva: " O wise hare, be thy virtue known throughout a whole aeon." And squeezing the mountain, with the essence thus extracted, he daubed the sign of a hare on the orb of the moon.
Thus in the Wheel of Life, the Buddha is pointing to the moon (wisdom) and the rabbit (bodhicitta), two elements which are needed to end cyclic existence and to become enlightened.