In Dark Souls, what boss is found at the bottom of the Depths?

In Dark Souls, Gaping Dragon is the boss found at the bottom of the Depths.

The Depths are a sewer-like area beneath the Undead Burg, reached after defeating the Capra Demon and opening the lower-city route. The zone contains rats, basilisks, thieves, and a large network of flooded passages before the boss arena.

Gaping Dragon is a grotesque creature whose enormous open torso resembles a mouth. Its attacks include lunges, charges, and corrosive vomit, while its exposed head and body create opportunities for close-range damage. The arena also contains a rescued merchant, Domhnall of Zena, outside the fog gate.

Players sometimes confuse this encounter with the Hellkite Dragon on the bridge or the Undead Dragon in the Painted World. Gaping Dragon is important because its victory yields the key needed to enter Blighttown, continuing the early Lordran route.