Newt spleens were the spleen of the newt. They could probably be used as a potion ingredient, or had some other magical use, as they were owned by at least one Hogwarts School of Witchcraft and Wizardry student. At some point in wizarding history, a student passing through King's Cross Station either entering or leaving Platform Nine and Three Quarters dropped a suitcase filled with newt spleens on the station floor, attracting the attention of Muggles. Undercover British Ministry of Magic, who patrol the station to deal with incidents like this, presumably modified the memories of those who witnessed this event.
- Pottermore (Mentioned only)