The Moon Dragon is one of the three rare dragons in DragonVale, next to the Rainbow Dragon and Sun Dragon. Same as the Sun Dragon, the Moon Dragon needs both the Cold and Lightning elements to be bred. This can also be done many ways, the plain Cold+Lightning, the popular Fire+Storm, and then some others, Cold+Firefly/Scorch, Mountain+Sonic, the founder got his with Frost Fire + Lightning and the also popular Blue Fire+Crystal.
Many people say that the time that the dragons are bred can affect if it is a Sun Dragon or a Moon Dragon. For some people, it is that way (breed at night for Moon and day for Sun). But for other people, it's the exact opposite.