The Waterbuck (Kobus ellipsiprymnus) is a large antelope found widely in sub-Saharan Africa. 
There are 13 subspecies that are grouped under two varieties: the Common Waterbuck (Kobus ellipsiprymnus ellipsiprymnus) and the Defassa Waterbuck (Kobus ellipsiprymnus defassa).
The Defassa Waterbuck is found in West and Central Africa while the Common Waterbuck is found in East and South Africa. 
The distribution of the two slightly overlaps along the Rift Valley in Kenya, Tanzania and Zambia.

The Common Waterbuck and the Defassa Waterbuck are remarkably different in their physical appearances. 
The main difference is that the Common Waterbuck has a distinctive white ring of hair on its butt while the Defassa Waterbuck has a solid white patch instead of a ring.
Also, the Defassa Waterbuck has a longer tail while the Common Waterbuck stands taller than the Defassa Waterbuck.

Defassa Waterbuck. Amboseli National Park, Kenya, 17 January 2008