Since Ra's earthly manifestation was the Mnevis bull, the three of Anubis as son, the Mnevis as father, and Hesat as mother, were identified as a family triad, and worshipped as such.
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).
Mnevis |