The beginning of the world was a very busy time, and there were many things to do. There was everything to do. So the Creator brought forth many helper gods to take care of all the little pieces. And these had helpers also, and so on. That is why there are so many gods and each is powerful within its special realm and the tribe must honor and attend to each. Somewhere in the middle of the great twining tree of gods, like a vine running from the canopy to the roots, was Nyaminyami. He was the god of rivers and streams, and this was surely an important task amidst the whole great twining of nature.
When the animals were created each animal spirit delighted, each in their special properties, of swiftness or stealth or ferocity. And they luxuriated in their material raiment, fur or scales, claws or hooves. All except the hippo. It was disappointed in its short stubby feet and smooth skin wrapped so wide around its immense bulk. The stones of the earth hurt its feet and the sun up in the air burnt its delicate skin. So it thought and thought and came up with an idea. Then it went in search of Nyaminyami.
One day while Nyaminyami was dozing in the shade of a tree overhanging a river, the hippo ponderously walked up to him along the bank on its short stubby feet.
"Excuse me, God," said the hippo nervously. "I have a problem that I hope you can help me with."
"I would be happy to try," replied Nyaminyami, for he was in a good mood at the moment, though he was known for sudden changes.
"Well," said the hippo. "As you can see, I am very fat, and my feet are stubby and my skin is quite sensitive. The sun up in the air burns it quite badly and the stones of the earth hurt my feet. Don't you think I would be better off with some fur or scales to protect my delicate skin?"
"What! You would look absurd, a great waddling ball of fur. And you are quite powerful enough already, what with your bulk and your great wide mouth. With scales you would fear nothing and be able to lord it over all the other animals."
"Oh no," protested the hippo. "I am quite peaceful. But you are right, I am better off with my smooth skin. And I suppose nothing protects better from the heat of the sun than the shade of the wide acacia tree, or soothes the feet better than the fine dust of the earth."
"Not even remotely close!" retorted Nyaminyami. "The coolness of my waters is beyond compare, and the smoothness of my river mud is the finest balm."
"If God says it, then it must be so," said the hippo, seemingly impressed, though secretly smiling, for this was precisely where it had wanted the conversation to lead. "I would be happy indeed to spend my days underwater, wallowing in the cool soft mud of the river bottom."
Nyaminyami paced back and forth while the river behind him crashed in waves and eddies and holes. He had said he would help… But to let this great big hippo in his marvelous river?! "Out of the question," he said. "After all the time I spent creating lovely lithe fish? Look at the size of your mouth - you would gulp them all down in one swallow."
"No, no, not me," cried the hippo. "I eat only plants. I would keep your river bottom clear so it can run smooth and free."
"Ha," Nyaminyami shook his head. "Who would believe that, seeing your great big teeth? How could I trust that you would never eat my marvelous fish? Go away."
The hippo waddled despondently off. As its feet ached from the stones of the earth and skin peeled from the sun up in the air it thought and thought and came back the next day. "I will prove to you that I will never eat the fish. Many times a day I will open my mouth very wide, wider than any other animal, so you can look all the way in and see there are no fish inside." It thought this a very good promise.
But Nyaminyami would have none of this. "Not good enough. You might just have gulped them down very quickly." Again the hippo trundled off, bemoaning its feet and skin. The next day it was back, looking quite miserable as it picked its way along the stony bank, wincing with each step of its battered feet.
"I have only one thing more I can promise," the hippo said hoarsely. "I will promise not to shit in your river. All day long I will hold it in, and every night, once the sun has gone away, I will leave the water and come up onto the shore where there is smooth grass that will not hurt my feet. I will shit on the open ground as I wave my tail to spread it far and wide. Then you, God, can look easily and see that there are no fish bones."
By this point Nyaminyami was feeling a bit sorry for the blistered sunburnt hippo and the extraordinary promises it made. "For so long as you do these things I will set you as the greatest of the river creatures, and let you dwell in my waters and rest in the cool mud eating water plants." Just so, from the earliest of days the hippo opens its mouth very wide so that the god can see in, and comes up on grassy shores every night to spread its shit for the god to check. And that is how the hippo got its nature: just so.