In late Egyptian mythology, Mer-Wer (also spelled Mnevis) was an aspect of the chief god in the region of Heliopolis, Atum-Ra. The origin and meaning of its name is currently unknown.
Mnevis was identified as being a living bull. This may be a vestige of the sacrifice of kings after a period of reign, who were seen as the sons of Bat or Hathor, the ancient cow deity of the early solar cults. Thus, seen as a symbol of the later sun god, Ra, the Mnevis was often depicted, in art, with the solar disc of their mother, Hathor between its horns.
A suitable bull was selected from the area, said to be the living Mnevis bull, and was taken to a special temple, where it was worshipped and its movement used as an oracle. Since the fertile soil of the Nile was so black that the word for black (Khem) became the Egyptian word for Egypt, and bulls in this region had a tendency to black colouring, the bull selected to be the Mnevis was traditionally completely black, thus being referred to as Kemwer, meaning great black (one). When a completely black bull could not be found, they chose one that was completely white, in contrast.
Eventually, it became identified as another form of the increasingly more popular Apis, which had become considered an aspect of Osiris when his cult became prominent.