The Flume Horror (Fluminicola horribilis) was a monster rarely seen in Equestria until ponies took up civil engineering and greatly increased the availability of suitable habitats for the creatures. Flume Horrors are creatures which haunt small waterways, being particularly attracted to aquaducts, abandoned mills and poorly maintained dams. An eradication campaign in the 1400s AE, in which such structures were refurbished or demolished, greatly reduced the incidence of deaths attributable to Flume Horrors, although it was occasionally suggested in whispered rumors that the Equestrian state had another motive: elimination of the supply of alchemical flume, a deadly magical reagent.
The Flume Horror is the free-moving stage of the life cycle of its species. When it mates with another Flume Horror, it seeks out an ideal habitat and metamorphoses into its sessile, reproductive form - a large carnivorous plant-like creature with tentacles. The sessile form releases cysts (each about a centimeter long), which are inhaled by animals. The infected creatures eventually become Flume Horrors if not treated, their sinister transformation usually occurring with surprising rapidity after a prodromal phase consisting of personality changes, weight gain, fever, and an obsession with waterworks and typical flume horror habitats.
Alchemical flume is made from the immature reproductive cysts of the creature.