Mokk can be found in the middle left of Inkwater Marsh after Ori's fight with Howl. Upon interaction with Ori, he will give the Quest A Little Braver, in which he requests for a Howl fang. Later, he can be found in Wellspring Glades, where he tells Ori to spread his tale of his fight with Howl.
Are you a spirit? Then tremble, spirit! You stand before Mokk the brave! It is not very convincing, I know. But I am brave. Or I would be, with a trophy! A trophy like...like a Howl skull! Or just a fang, if that's too heavy. I heard the Howler fighting to the east. Maybe it dropped one?
After giving Howl fang
Ooh. A real Howl fang! None will doubt Mokk the Brave now! This is for you. I am brave but I am also generous.