This is the Gaping Dragon
Remember to take out the Channeler before you fight this boss. There is a Channeler located near a group of large rats near the top of the wall overlooking the eventual boss fight area, beyond a small fog gate. If he is not killed prior to the fight with Gaping Dragon, the Channeler will continuously fire soul arrows at you, and may also buff the Gaping Dragon if it gets close enough to him, making it significantly stronger.
Additionally, not only does keeping the Channeler alive allow him to buff the boss, but also allows him to fire spells down at you from above.
The Channeler's combat spells are not particularly dangerous by themselves, but have a frustrating tendency to hit you in the head from above and stagger you just as you are attempting to either run at or roll away from the boss.
Fortunately this enemy does not respawn once killed, so you won't have to deal with him every time, if you die to the boss.