They are monster from Sillent hill everything have or is made from blood haha
Depending on whoever enters Silent Hill anyway. In the Second game it's made clear that not everyone sees monsters, the Third game might further solidify that if the nightmare horny guy (forgot his name) is truly implying that Heather is seeing people as monsters. Idk if it's actually ever addressed in the other games. The two I mentioned I just recently replayed so my memory on them is fresher than the others.
I'm not sure if I want to believe that Silent Hill is a ghost town that manifests monsters, or if there are residents but depending on the person being subjected to Silent Hills torture sees them as monsters or not.