Origin[]
The Rain Man is an alleged demonic entity coming from various conspiracy theories surrounding the music industry, either as an alias for the Devil himself or a separate demon. It is said to corrupt those who want to gain fame in the industry by "raining" material wealth upon its subjects and giving them success, in exchange for their servitude towards the demon.
This entity is supposedly alluded in different songs, like Eminem's songs in his album "Encore" and Rihanna's "Umbrella". The latter is the most infamously prevalent "evidence" the theorists use to support the existence of the demon. In the music video, Jay Z sings "Rain Man is back with little miss sunshine, Rihanna where you at?", to which Rihanna responds by singing "You've had my heart, and we will never be worlds apart, you're apart of my entity, here for infinity." The theories claim it debatably converts from a love song to Rihanna swearing her allegiance to the Rain Man, who enters her body and contorts into a visual representation of a goat-like beast within a triangular prison.
All these claims weren't confirmed in any way, and it's highly likely these are no more than random coincidences believed to be connected in one way or another.
Copyright[]
Since this entity was created by conspiracy theories rather than an orphan work, it's a legendary creature and thus a public domain character. However, its "evidence" comes from copyrighted material such as music videos and lyrics, so only the demon's concept can be used.
Furthermore, MGM owns a trademark for the name "Rain Man", so the name can only be used inside a work and for this specific entity in order to avoid confusion with the 1988 film.