This taxon is Not Recognised as a species by BirdLife International.
Horizocerus hartlaubi and H. granti (del Hoyo and Collar 2014) were previously placed in the genus Tockus and lumped as T. hartlaubi following Sibley and Monroe (1990, 1993).
BirdLife International (2016) Species factsheet: Tockus hartlaubi. Downloaded from http://www.birdlife.org on 05/12/2016. Recommended citation for factsheets for more than one species: BirdLife International (2016) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 05/12/2016.