@Onychorhynchus coronatus Agree with you here. I might be totally wrong, but the face of the mangabey doesn't have this shiny leather aspect. this is the skin aspect of the drills. I would guess this is a juvenille female drill.
@David Matos Mendes Yes, it is definitely a drill. It has that long dog like muzzle, the dark pigment of the face and the characteristic "lipstick" lips.
