The Grey-cheeked Mangabey (Lophocebus albigena) is an Old World monkey found in the forests of Central Africa. 
The status of this monkey and whether its originally recognized three subspecies are indeed subspecies or should be regarded as full species, is still unclear.
The one occurring in Uganda is the Uganda Mangabey (either Lophocebus ugandae or Lophocebus albigena ugandae; see 4th & 5th photo) is found only in Uganda and in the Minziro Forest Reserve, just over the border in Tanzania.
The Uganda Mangabey is rather smaller than the Grey-cheeked Mangabey (Lophocebus albigena) and has a smaller skull.

1st, 2nd & 3rd photo: Bigodi Swamp, Kibale National Park, Uganda, 26 January 2025
4th & 5th photo: Bugoma Jungle Lodge, Bugoma Forest Reserve, Uganda, 9 September 2025