Please help - Yaoi Manga

I've been trying to find a yaoi manga (probably an old one) for a very long while to no avail and it's really bugging me that I can't put my finger on it. All I can remember is a random side story to the main story though (or a one-shot from a collection maybe).

It's about a demon/supernatural being of some sort getting lost (this may be very wrong but I think it was stated that he "fell out of the sky" or something) and being captured by a man (researcher/professor of some sort). He chains and confines the demon (the demon probably injured or in a weakened state) and plans to use him as a scientific discovery to become renowned, famous, rich etc. The researcher has an assistant/apprentice/student working for him and everyday, the cute assistant is kind to the demon and brings him his food etc. Time passes and I believe they become attached to each other from their regular interaction. One day, the researcher is being aggressive/abusive in some way to his assistant and the demon sees and becomes livid, breaking his chains in order to protect the assistant.

This is as much as I can recall. It's quite vague and I have no idea if I misremembered any of the details or if this was all a figment of my imagination, but please do help me find this manga, if you can. It would be a huge relief to my mind. I really appreciate it.