Blacktail is a living treasure, and the object sought by a merchant of the trading clan.
A really fine bull, who sires more calves than normal.
Your clan already possesses Blacktail the Bull at the start of the game, though he may be bought/sold like any other treasure.
Note: Blacktail the Bull is a treasure with a finite lifespan. He seems to live 8 or 9 years.