It looks like the Angolan Colobus is found in Tanzania, Kenya, Uganda, Congo, etc. If it is found in Angola at all, it seems to have less of its range there than in other countries. So why the “Angolan” colobus?
More or less the same reason why the Baltimore oriole, the Virginia opossum, the Lapland longspur, the Manitoba maple, and a wide variety of other animals are not exclusive to the area with their name: it marks where they were discovered, not the extent of their range.