You know, it's thought that Archibald had turned his good friend Paarl into a darkbeast, to more easily study it. It's the reason why Paarl doesn't attack the player upon approach, and is deep within old yharnam, likely to hide from curious blood-addled hunters...

Yeah, I´ve heard about that theory too, but personally, I like to think that Paarl is a far more ancient creature, as it is basically undead, unlike other darkbeasts we meet. Archibald probably captured (or tamed?) it for his studies, but it´s origins are shrouded in mystrery and may even be traced to the dephts of the Pthumerian Labyrinth, or even to the forgotten land of Loran...