The horn shape and its sex dimorphism are compared in museum specimens of all geographical populations of the dama gazelle (Nanger dama). Departing from a putatively ancestral morphology prevailing in the west(-central) Sahel zone, a forked cladogenesis is inferred producing the eastern horn type of the red-necked gazelles (N. d. ruficollis) in Sudan and the morphotype of the mhorr gazelles (N. d. mhorr) in the northwest Sahara. The mhorr-horn type is phylogenetically derived as an autapomorphic novelty evolved by a mechanism driven by female sexual mimicry of the male model. Insufficient museum series from the subspecifically vaguely defined populations in the Sahel belt preclude the recognition of one (or even more?) additional horn morphologies in this intercalating region from Senegal to Niger, but any perhaps possible, further type(s), such as a potential horn morphotype in Damergou, Niger, would be weakly differentiated from the ruficollis-horns. Horn morphology suggests moderate subspecies-specific originalities in the fighting behaviours, differing in the intensity of ramming (N. d. mhorr) versus sparring (N. d. ruficollis). The slightly augmented surface area of the eastern morphotype could facilitate thermoregulation, by horn-mediated export of body heat, in the hyperthermic and hyper-arid desert biotope of N. d. ruficollis. Every morphotype is broadly sex-dimorphic, but this dimorphism is reduced in the mhorr gazelles. Andromimicry of female mhorr gazelles, mimicking the sexual phenotype characters of males, is proposed to explain both the diminished sex dimorphism of the horns in this subspecies, and its conspicuously gaudy display and shiny signal coat colours. While the horn ontogeny in female addra gazelles remains unstudied, female mhorr gazelles continued horn length growth during adulthood until old age, but males apparently did not or less so, further raising the complexity of horn shape evolution.