Ben Hibburd’s review published on Letterboxd:
Whoever came up with the title of this film should be fired. "Horse Girl," really? That's the best you could come up with? It's awful!
As for the film I'm struggling to figure out what this film was trying to accomplish? It's vague about its ideas and what it wants to say. The film focuses on a young girl, Sarah (Allison Brie) who lives a quaint but wholesome life. She works at a hobby/craft store, watches her favourite supernatural show and has no friends of substance, in fact most people seem to want to distance themselves from her.
What could've been a touching off-kilter look at mental health dives head first into full William Friedkin "Bug" mania. This would've been fine if there was some background information on her illness but there's none, leaving the viewer guessing which I felt was a mistake.
Alison Brie gives the performance of her career. She is the glue that holds this film from collapsing under the weight of itself. In fact, she's too good for the material she's given to work with. In the hands of Sam Esmail or David Cronenberg I could only imagine how good this could've been. Unfortunately a weak script and bland direction creates an almost lifeless experience.
This film had so much potential, it could've gone down multiple avenues and explored mental health in a profound way from the symptoms, delusions, friends and family, to the lack of funding and resources. However, the film only dabbles its little pinky in each without offering any real insight or depth.