Well, that's the hard part. They show up in several different anime/manga stories, and they're names and characteristics are consistent /enough/ that I suspected they must have a common source. This, indeed, appears to be the case.

I tracked them back to Abe no Seimei, through onmyoudou to China, and there it all unravels into guessing games. They're definitely associated with Chinese astronomy/astrology, but they seem to go back to pre-Han. I got side-tracked for a bit into Buddhist sets of twelve, but I think that's pretty much coincidental and not directly related. So I'm fishing around in this book and that, trying to match the characters of their names with various sky charts and star legends.