As she speaks to the therapist (Jay Duplass), it seems like she's taking steps toward getting help. But that all changes when she's left alone for the night. In a disorienting 10-minute sequence, one version of Sarah leaves her ward while another stays behind watching from above. She goes on a hallucinatory journey where reality and her personal fictions blend into one another. Cloaked in a pastel, ninja-style costume, she moves through slightly altered locations from her life, eventually ending up in an alien realm witnessing shadowy figures probing her own body. When she wakes up, she's in the bed of her roommate at the hospital and they are holding hands.
The girl (Dylan Gelula), the same one who has been appearing in her visions, is initially horrified, but as they talk they come to realize they've shared some of the same delusions. It's enough proof Sarah needs to decide that what she's experiencing is real. After she's discharged she goes home, makes herself up like her grandmother, and goes to the stable where she absconds with Willow. They walk to a golf course where Sarah drops Willow's lead, takes off her shoes, and lies on the ground. Sarah's body is lifted aloft by a light in the sky. In the last shot, she's gone as Willow patiently grazes.
Brie and Baena designed this ending to be puzzled over. In an interview with Vulture, the star said, "If there’s not a Reddit message board about this movie, I would be shocked and frankly appalled." In a move that's sure to stir some debate, the filmmakers insert details that lend credence to the idea that perhaps what Sarah is experiencing is happening in reality. For instance, in the first scene of the movie Sarah's friend at the craft store Joan (Molly Shannon) looks up from their conversation to see just the tail end of a horse walk by. By the end, it appears that she was watching Sarah, the person to whom she was talking, walk Willow to their ultimate fate. So does that mean Sarah is abducted by aliens at the end? Possibly, but it's also more likely that we're watching whatever is happening in Sarah's mind rather than what's happening in the real world.
For the first three quarters of the film, Brie plays someone fighting against herself. By the end, she exhibits an unsettling serenity, having embraced the theories that once disturbed her. Is this liberation for Sarah? Or is she lost, giving over to her disease? Horse Girl doesn't answer these questions; it wants to leave you in the discomfort of not knowing.