Page #45: Your Werewolf Name
It is also often shortened to Menu anyway! (Also, Menuo and Menness). If you're wondering how Charby happens to just know this random fact, he's been around a long time and doesn't have electricity in his old home so he visits the library after-hours. He doesn't know everything, of course.
Truth be told however, it was a happy accident that Menu happened to be the name of a lunar god! But it just makes sense to me that's why Charby would pick that as a name for him. And kinda yeah, that's why he picked that out of all the possible names. He knew what he was doing.
At lease he didn't name him Moon Moon! (Θ ヮΘ)