I'd watched the anime between 2008 and 2012, although I feel it was closer to 2010. The anime was originally Japanese but had English subtitles, I'm not sure it had even been dubbed. The anime had been available on Crunchyroll, although is no longer available.
There’s a boy who at night turns into a beast because of a gem/crystal inside him. I believe that the first episode shows him turning into a monster in black. Later on, a girl and her butler meet the main character trying to help him but he rejects their help. The girl explains to him about the power of the gym/crystal that he posses and he still angrily rejects them and forces them to leave.
He then starts to feel a pain from his forehand and grows a large crystal/gym like horn and once more turns into a monster that kills and hunts another monster like itself. He eventually gets help from the girl and using the power the girl gives him, he’s able to control his power and help mankind fight off these monsters. I think near the end of the anime he fights a Medusa like monster or something along those lines.