I am not an experienced fan of anime so I don’t know where to start, I can’t Google it and while I am sure I watched it on YouTube searching there reveals nothing.
Anyway:
This is likely not obscure and not very old. It was about a young guy (18-25) who one day has his hand replaced by a hungry ‘Spawn-like’ alien. The story is then this kids life, school, girls and him hiding his deformity and the fact it wants to keep eating people.
Parasyte is fairly recent as things go. I thought we were gonna talk about stuff from before the Dark Age of Technology like Crystal Triangle (1987), The Humanoid (1986), or California Crisis (1986).