The rainbow monster in African mythology myths and stories is a terrible animal that lives in water and comes out under the cover of darkness to devour other animals and even people.
One such monster lived in the Lake Naivasha and it swallowed many Masai cattle before many young men managed to catch it and destroy it.
Those rainbows seen in the sky are not the monster itself but the reflections of it. (Gikuyu myth, Kenya) A.G.H.
Savil, Sheila. Pears Encyclopedia of Myths and Legends. “Western and Northern Europe, Central and Southern Africa.” London. Pelham Books Ltd. 1977. p. 157