not a true story but there are obviously many examples of this type of thing going on throughout history (in terms of feral children, of course no one wants to think that anyone could be capable of intentionally manipulating someone, particularly an innocent child into this animalistic state but it absolutely goes on and is happening somewhere right now; dehumanization is an extremely common form of abuse and an essential mechanism for programming). Let's hope the forthcoming organized chaos does not allow something as horrific as the colossal level of dehumanization that occurred during the Nazi era to happen again (I'm not very optimistic [these things are cyclical by design, we're due another world war]).
Bird cage/feathers/masks/etc [1920's-30's Germany]
Jeff Goldblum plays circus entertainer/ringmaster 'Adam Stein' using his own daughter (Ana Geoanna according to a NY Times review, I can't find any info on her at all) in the knife-throwing show (obviously quite traumatic).
The story jumps from him in the mental asylum, where his damaged self is shown as he can only 'get off' apparently when dehumanizing others (symptom of this kind of this especially twisted psychological trauma) which he does to his love interest in the institution.
The dehumanized kid 'David' is shown chained and collared (and acting 100% like a dog having been dehumanized). He is played by Tudor Rapiteanu whose other role has him in a short film being used to traffick drugs (like young trafficked/mk slaves in reality [as they're not suspected by the authorities for obvious reasons]).
Why So Serious?! He's been made into a 'Joker'. I haven't gone into much detail here (did not want to turn this into a long post), there's more imagery/themes worth mentioning in the film.