Creatures of THE Night
All right, progress.
"So you remember how changelings work? Human baby and a blood transfusion without their consent? The monster is sad because it's infertile, but still really wants kids. So, it breaks into the home of a newborn. One that hasn't had its first birthday yet, because old magic is ageist somehow. So the monster feeds the baby its blood, once every full moon, for a year. And because cryptids are teeming with magic, the baby mutates. Could become a full cryptid itself on one side of the scale or just a magical human on the other end. Either or, changelings always come with a mark. A mark that's like a type of genetic ID until their parent monster comes to take their child home.