Once, the Common Waterbuck and the Defassa Waterbuck were considered two separate species. Nowadays, modern taxonomists consider them as a single species. 
There are thirteen subspecies which are grouped under two varieties: the Common Waterbuck and the Defassa Waterbuck.
The distribution of the Common Waterbuck slightly overlaps that of the Defassa Waterbuck along the Rift Valley in Kenya, Tanzania and Zambia.

Top & Left: Arusha National Park, Tanzania, 16 January 2008
Middle & Right: Serengeti National Park, Tanzania, 12 January 2008