Blackteeth

If you're close enough to see the color of its sharp, pitch colored teeth.Hesper had been too close. Along with five other children, she vanished without so much as a shred of clothing left behind. Ten years later she's found, soaking wet, on the side of a river, telling a story about monsters with black teeth. No one believes her, and she is forced to gather up the scraps of her life and move forward.
One of the other missing children's family does believe she has information she's not sharing and kidnaps her. The monsters discover where she's located, and they don't forgive slights because Hesper has slighted them terribly. She has to run for her life and when that doesn't work, she has to go on a painful and violent offensive to protect herself and others. The monsters don't go bump in the night, they go splash in the water.