Original image

step 1 of 4
Then 'L' for the lasso tool and roughly off around his nose.
CRTL+J to paste to a new layer then hid this new layer. 'S' for the clone tool and cloned out the original nose.

step 2 of 4
Put the nose back in and added another mask, did the same with the eyes.
Burned where the mouth was going to go.

step 3 of 4
Teeth from the horse image just warped into place and masked, neck made from the guy's forehead and copied, merged, burned, etc.

step 4 of 4
And that's it really, Bob's your Uncle, or maybe Aunt, if you're from Algeria ;)

Final result

