Here's why:
* Lanyon's Description: In Robert Louis Stevenson's *The Strange Case of Dr. Jekyll and Mr. Hyde*, Lanyon is portrayed as a staunch, rational, and scientific man.
* Witnessing the Transformation: When Lanyon witnesses Hyde transform into Jekyll, it shatters his world view and his belief in the natural order. This is a horrifying experience for him.
* Lanyon's Reaction: The text explicitly states that Lanyon is "terrified" and "sickened" by what he sees. He is so traumatized by the experience that it leads to his eventual death.
While other words like "disbelief" or "shock" might also apply, horror best encapsulates the depth and intensity of Lanyon's reaction to the supernatural and monstrous nature of the event.