Pale horse's balm
From Discworld MUD Wiki
It looks like:
A jar of pale horse's balm costs A$225, it looks like: Said by the gullible to be made from the fallen hairs of Death's own horse, this jar of rare balm, worth more than its weight in gold, is the only known treatment for the similarly rare horse ailment known as resurrection sickness. Its true recipe is a very well-kept secret. It is full of pale horse's balm.
It's been said to hold enough for 4 applications.