Darkbeast Paarl is an optional boss who you may or may not encounter during your playthrough of the game. He is a sub-boss of Yahar'gul, Unseen Village, and he guards the door that connects it back to Old Yharnam. You can encounter him early on in the game (as your 5th or 6th boss) by letting one of the "death eater" enemies who carry sacks on their shoulders kill you. This will cause you to wake up deep inside Yahar'gul, and one of the ways to escape this area is to kill Darkbeast Paarl. However, he is extremely challenging when you encounter him early in the game, so it is recommended that you avoid this if possible.
If you did not encounter him early in the game, you can find him later on by going to the lamp at the bottom of Yahar'gul (where you fought The One Reborn) and backtracks into town a bit. He isn't difficult to find if you search around the lower areas of the village. See the following video guide: