Request #234

Hoping someone remembers this book, I read it in the late 90s but I feel like it would’ve been published in the 80s. I only have a few vague memories…
A girl goes on a camp, meets this popular group of beautiful girls and they sneak out at night to go to a bar or something and hang out with handsome guys. Then after a while, the girls tell her they have to sacrifice her to a mythical beast (a sentinel maybe?) And that’s how they’ve stayed young and beautiful for so long…
The book was a small paperback, black and had a white horse on the front who maybe had red eyes???
Hope someone can help, it’s been driving me mad