Howler Monkey

It looks a lot like a Mantled Howler monkey, but I'm not that sure! There is some Howler subspecies I don't even know about it, but what you said Yasser is totally true their howling is really amazing!

Totally, I remember first hearing their cry at night on my trip through the Amazon. They're actually very shy and being able to spot is a real privilege. Your spotting looks very similar to one I saw, except the colour is a bit different http://www.projectnoah.org/spottings/2586007

These monkeys are great! They're pretty small and non-aggressive, but when it's night and you hear their howling for the first time, it sounds like it's coming from an 800 pound beast that wants to eat you, haha!