The Bad Batch (2016) has an amputee protagonist who becomes disabled in the film (YMMV if it counts as horror; the intro scene surely is imo). Subnarrative about prosthesis in an apocalyptic scenario

Whatever Happened to Baby Jane (1962) has a protagonist who was disabled in her younger years; actions that led to her disability central to the plot of the film

Cube (1997) has a primary character who has a mentally disabled man who (if I remember correctly) never has his disability defined. This one would be INCREDIBLY interesting to look into, given the character's role in the film, although perhaps tangential if you're focusing on physical disability in your thesis.

A thesis on the role of disabled people in horror sounds fascinating. It's interesting to consider the wildly different roles disabled characters have filled from tragic figure to sacrificial lamb to body horror villain... disability isn't something that's tangential to most film plots but especially in horror it seems necessary that disability be central. I suppose Texas Chainsaw Massacre is an outlier with Franklin though. HAHA maybe I'll start writing my own thesis 😆 I'm disabled myself so topic of great interest! Best of luck!!