The Horned River Serpent is a snake-like creature that has a jewel set into its forehead and a house named after it at Ilvermony.
- Isolt Sayre encountered a Horned Serpent who always gave the same message. "Until I am part of your family, your family is doomed." (Pm). Isolt named a house after the creature and still felt a sense of kinship with it, she could understand it, but not speak to it.
- When Gormlaith Gaunt came to Ilvermorny to attack Isolt and her family, she said the Parseltongue word meaning "wand sleep," which rendered Isolt's Basilisk horn wand core useless. The same word had a different effect on the wands of Webster and Chadwick Boot. Their wands had twin-cores made from River Serpent horns which began to emit a "low musical note" that wakened the boys and made them aware of impending danger from Gormlaith (Pm).