The Summer Prince — Carol Oates

On the eve of her
sixteenth birthday, Niamh sneaks out to attend the festival of Beltaine.
Not long into the celebration she encounters a mysterious young man.
When Niamh has to choose between running deeper into a dark forest with a
supernatural creature that shouldn't exist or facing an unknown threat
chasing her, Niamh discovers she will have to rethink everything she
knows about the Fae.