Once upon a time a troupe of foreign constables chased a beast all the way to Yharnam, and this is what they wore.
The constables became victims of the beast, except for one survivor, who in turn devoured the creature whole, all by himself.
The fable is a favorite among Yharnamites, who are partial to any stories of pompous, intolerant foreigners, who suffer for their ignorance. It makes the blood taste that much sweeter.
The Hunter's Nightmare: Near the bridge over the blood river with the Bloodlicker enemies, find a Beast Cutter hunter NPC and defeat him. Climb up the ladder, drop down to the next platform and walk across the wooden beams to a body holding the armor.