"Mgamu", a short story by Lord Dunsany in his collection The Fourth Book of Jorkens. The story is told in the Billiards Club by Jorkens, who is repeating the story told him by a man named Polder who encountered the sivver-verri.
"The sivver-verri," said Jorkens, "is described by the natives as a distinctly unpleasant beast. I have heard them describe him myself in Kenya. The Samburus know him and the Kikuyus, and of course the Wandoroboes. And even among the stout Masai I have met men that were afraid of the sivver-verri. When I found that the Masai were afraid of him, I knew that he was a very terrible beast. But the odd thing about all the stories I ever heard about it, barring the one I am going to tell you; and all the hints and whispers of him; was that no one had ever claimed to have seen the sivver-verri. It ws something that came very rarely to the huts of reed, and only by night, tearing its way in through the roof; and wherever it came it never left a survivor. All this I had heard; and so one day, when in the smoking-room of a ship in the Indian Ocean, going down the African coast, some men began to speak of the sivver-verri, I pricked up my ears, though I did not think it likely that I should hear anything new: on the contrary, I rather looked to telling them something they did not know, which I am afraid may be becoming rather a habit of mine, when conversation comes round to Africa."
[. . .] And I think I was about to tell them something that I had heard myself about this beast that guards his invisibility so successfully against every living native of Africa, when my breath was taken away. Literally taken away. I stopped breathing. A little man with a clipped grey moustache who had not spoken, and whom I had put down in my mind as a doctor in a small English village taking the one trip of his lifetime, said 'I have seen it.'
"'You have seen the sivver-verri?' we said.
"'I have seen it,' he said again And then he told us about the sivver-verri, the only man in the world who claimed to have seen it. So you see, things have been seen in Africa that I have not met. I merely mention it in case you should think that I claim to have met everything."
Again we made the right noises in our throats.
"Because that is not at all the case," went on Jorkens.
The title is the name of a witch-doctor in the story:
"'[. . .] And on the way I met a witch-doctor hanging about. He was the evillest-looking devil I ever saw. And he gave me a nasty look, making his face even more horrible and causing me to think that he was jealous of me. Quite a reasonable idea for me to have, because magic is what the natives had actually asked for, and I and my rifle must have seem to them very much like a man arriving with a machine-gun at a pheasant-shoot. For two reasons: the machine-gun would be far too powerful, and also ineffective.
[. . . .]
"'I asked them about the witch-doctor, wondering what they thought of his jealousy.
"'"That is Mgamu," they said.
"'But there are men in many countries, as I have often noticed, whom nobody talks about much; and Mgamu was evidently one of these.