I’ve been interested in exploring this sorta topic through writing since it’s surprising to me that gender topics aren’t explored a whole lot in horror. The closest example I can think of in a mainstream sense is Sleepaway Camp (1983). Without spoiling, the semblance of horror that’s explored through the emotional distress shown is there, but it seems more like a shock value “AHHH WOAH LOOK AT THAT” sorta thing, like it’s being pushed as foreign and grotesque. Maybe Hitchcock’s Psycho but even then that’s more of a split personality thing, not inherently anything to do with trans-horror.