These are stories which provide a mythological explanation for peculiar things in nature or certain events and customs of which the origin has long been forgotten. Two examples are why the Ethiopians are brown-colored (see: Helios) and the story of why the rhinoceros has no hairs (it had been on fire once and then jumped into the water; since then his long hairs have never grown back).
Some scientists do not refer to these as myths in the traditional sense, but regard them as later additions.