Darkbeasts are undead beasts. Their hides are surrounded by blue sparks.
The darkbeasts of Bloodborne are differentiated from other beasts by their ability to generate electricity, or bolt, from their hides and by being in nature undead. Paarl is discovered lying as if dead in the Graveyard of the Darkbeast, located at the entrance to Yarhar'gul from Old Yharnam. Paarl will drop the Spark Hunter Badge when defeated. Spark Hunter Badges were crafted in secret by Archibald, the infamous eccentric of the Healing Church for his friends, implying some connection to Paarl. Archibald was fascinated with the blue sparks that emanate from the hides of darkbeasts, and sought to recreate them in a form of inquiry that closely mirrored the methodology of the Byrgenwerth scholars. Archibald and his inventions were unpopular among his peers. Despite crafting what has come to be regarded as masterpiece; the tonitrus ('thunders' in Latin), the hunters of Archibald's time did not fully take to the device.
Paarl being a darkbeast clearly relates him to Archibald, as we know that Archibald was obsessed with artificially recreating bolt. The environment we find Paarl in also tells us something. All of Archibald's inventions can be found in and around Yarhar'gul (the Tonitrus, the Tiny Tonitrus and bolt papers). The Tonitrus and Tiny Tonitrus were unpopular among hunters which explains why they would not be found outside of where they were invented, the bolt paper was a more popular invention and can be found throughout the world of Bloodborne. This evidence strongly implies that Archibald was working in and around Yarhar'gul during his lifetime.
The gate between Yarhar'gul and Old Yharnam can only be opened from the side of Yarhar'gul, implying that it was therefore closed from that side. The area Paarl is found in can only be reached through the cells of Yarhar'gul. There is a tunnel that appears to have been artificially created found in the back of the complex leading out to the graveyard. The existence of this tunnel seems very incongruous with the rest of the prison cells. If it was meant to be part of the building it would likely have a more constructed and designed look to it. But since it does not, it is a likely assumption that the tunnel was created forcefully, perhaps by the frantic and enraged claws of Paarl as the darkbeast tried to escape its cell.
All of this evidence adds up to tell this possible narrative:
Archibald, obsessed with artificially creating the blue sparks of the darkbeasts, likely needed a darkbeast to study to create the sparks. So, voluntarily or not, Paarl likely became his test subject. The experimentation of Archibald eventually transformed Paarl into a darkbeast, who subsequently broke out of its confines and clawed its way into the entrance of Yarhar'gul, perhaps hoping to escape. It is likely that the gate was already locked, and that after this effort Paarl 'died'. But due to the nature of darkbeasts, Paarl continued to live on as skin and bones despite having rotted away.