I have been afflicted with a high degree of horror of transformation, which I control as best I can because it's not in step with my circle of friends or, indeed, many of my own headmates. "Body horror" has always squicked me more than other types of horror, and it's been getting in the way of my cinematic interests. I suspect that this is a common problem in U.S. culture, which is suffused with Christian-tinged ideas about the body as a temple and the God-ordained fixity of all creation. Evolution horrified the Christian hardliners with its suggestion of fluid changes between forms, and they rejected it; physical transformation of the body terrifies them even more.
In order to understand my own horrors better, I have turned to one of the writers who has best evoked it, the Welsh weird-fiction writer Arthur Machen. Machen dabbled in occultism but, somewhat like H. P. Lovecraft (who drew from Machen's work) he was horrified by what he saw beyond the veil. Unlike Lovecraft, unfortunately, Machen tended to sexualize his horrors and thus weakens his fiction. His most famous work, The Great God Pan, superbly evocative in many passages, frequently sinks into mere hand-wringing about unspeakable obscenities. Machen rather suggests that Helen Vaughan's greatest problem is that she's horny and kinky.
Thus my favorite Machen story is The Novel of the White Powder, a self-contained excerpt from a larger work called The Three Impostors. The style of this book is highly discursive, with shady characters frequently stopping to tell short tales that are thematically pertinent but otherwise apparently irrelevant to the main narrative. This framing-device gives an odd quality to The Novel of the White Powder, which is related in a tone of extreme urgency and sincerity. Its horrific power comes partly from this mood of utmost seriousness—and yet there's an excellent chance the story is complete bullshit, told by an unreliable narrator playing games with her audience.
cw: extensive discourse on Machen's The Novel of the White Powder, with copious spoilers