"On the outskirts of Assin Manso is the Slave River, where the kidnapped and shackled Africans have their last opportunity to wash down after uncountable number of days trekking by foot through thick forests and dusty savannah tracks with little or no food on a journey to the coast. Final inspection and categorization of stolen Africans were then made before they got to the final transit point in the infamous dungeons of the Castles and forts on the coast. Those who were too weak to continue were murdered and buried on the banks of the river. It is believed that the reddish color of this river is due to all the blood spilled into it over several years of slave trade. In the Akan language, donko means slave and nsuo means water. Therefore Donko Nsuo literally means Slave Water hence its name Slave River. Though this tragic part of history was never documented, the legend has passed on through generations thus the local people have always called the stream Donko Nsuo." (source)
The red color of this demon represents the blood that was spilled into the water. This demon arose because of the tragedy that occurred at Donko Nsuo.