How rare is a moon dog?

Could you elaborate on the rarity of a moon dog? Is it a frequently observed phenomenon, or does it occur only in rare and specific circumstances? I'm curious to know if it's something that most people are likely to witness in their lifetime, or if it's truly a once-in-a-blue-moon sighting. What factors contribute to its rarity, and how do they come together to create this unique optical illusion? I'd appreciate a concise yet detailed explanation of the conditions necessary for a moon dog to occur.