Taking inspiration from nWoD, I’ve been running my own humans-turning-into-monsters setting with Fate, using a special 5-pt consequence track called Humanity, which players can use to get event-based boosts, by taking on aspects that represent their physical deviation from human baseline. It also acts as a limiter for their magic skills, first aspect grants 1 rank for a magic skill, second grants two ranks (so they could now have 3 in one, 2/1 in two, or 1/1/1 in three), etc.
Eventually, Evil Hat is supposed to put out a horror book for Fate, I’ll be watching that carefully.