The Fakastr are a race of semi-intelligent, massive, brutal bull-like creatures who grow limbs back after they have been hacked off, with a single exception. Their horns. The horns, when severed, remain alive and the Fakastr see it as a great dishonour to lose one. They always attempt to retrieve lost horns, escaping and travelling miles to seek out the owner and punish them. They can sense the presence of their horn and instinctively follow it, sometimes across continents…
The Fakastr are used by the people of Ichyria as entertainment: there are fighting rings where men contest the beasts and try to render them harmless. Of course the advantage is that any lost limbs grow back and the same Fakast can be used for many fights.
It is seen as a great prize for a Toreador-Warrior to hack off a horn of his Fakast, and Fakastr with no horns left are generally considered no good for the fighting ring.
Of course this is only known in Ichyria. In Malista, thousands of miles away, a severed horn has come into the hands of thieves and confidence tricksters. It has the strange property of filling up with a sweet red liquid when left overnight (of course the horn is still alive, so its fibres are still producing blood*). This they sell to the PCs as an “everful drinking horn” at a high price.
Can you imagine their faces when the Fakast turns up?
The item will eventually regrow into a full monsterous critter if left alone.
As an added curse you could make any drinker of the horn-blood gradually transmogrify into a Fakast.