Let's take a look at what information we have from the artbook.
Translated pictures from the official artbook courtesy of the wonderful GlitterBerri!
So we know that he was in a cave, and hung from the stalactites on the ceiling. There are a few caves in the game...but none of them have many stalactites, or are nearly large enough to house monkey, who appears to be a fairly big colossus. So where on the map could monkey have been?
...I really don't know how no one else has noticed this yet. (At least as far as I know they haven't. feel free to correct me if I'm wrong XD)
Look at this close up of square I-5 on the beta map -
Anyone else see the Gigantic cave on that beach? If not maybe this will help!
Still not convinced? Well, not for long!
Voila! one cave, large enough to house a whole colossus! In fact...
what is that on the beach...
Could that actually BE monkey?! It's hard to say for sure, but
it's a possibility! :)
Oh, and anyone remember that unused track on the soundtrack "Voice of the Earth"?
Gee, I wonder where that might've played? XD
I've had some suspicions about the land above the cave in I4 being the "Wetlands" that another of the unused tracks went with too. And if that's the case...well, I don't know whether to chalk it up to coincidence or Team Ico's amazing attention to detail but, if I remember correctly stalactites, stalagmites and many other cave formations, are formed by dripping water over a long time. This would have been one of the only caves in the game with formations, and if the wetlands are indeed above it, then one of the only caves that would theoretically, have the water supply around to cause said formations. The other cave with formations is the little one on the way to the 14th colossus Cerberus, and it has a good bit of water too.
So that's all I've got on this subject for now. I'd love to hear anything you've got to say so please, feel free to comment! Tell me what you think! Don't hesitate to correct my mistakes if I've made any! XD