Wikipedia
Ceratogyrus is a genus of spiders in the family Theraphosidae (tarantulas). Found in southern Africa, members of genus are commonly called horned baboons, for the foveal horn found on the peltidium in some of the species within the genus.
Ceratogyrus is readily distinguished from all other African theraphosid genera by the combined presence of a retrolateral cheliceral scopula composed of plumose, stridulatory setae and the strongly procurved fovea. The fovea is typically strongly procurved and in some species surrounds a distinct protuberance. this protuberance may take the form of a simple posterior extension of the caput, a low-set plug or a prominent, discrete conical projection. All Ceratogyrus species possess a pale yellow anteriorly placed, transverse, sub-abdominal band. This feature is not distinct in other Harpatirinae except Augacephalus junodi. The absence of dense, ventral femoral fringes on the palpi and legs I and II distinguish Ceratogyrus spp. from female A. junodi.
C. marshalli features the biggest horn, where it stands straight up about 1 cm. There are several probable functions for this horn: according to a study by Rick C. West in 1986, it provides an increased surface for the attachment of the dorsal dilator muscle, which aids in drawing in liquefied food into the sucking stomach at a faster rate; this way, the spider can retreat to a safe place faster. It also increases the area for the midgut diverticula to expand during times of nutrient and water availability, analogous to a camel's hump, helping it to survive in its arid habitat during droughts.
C. darlingi (often still known under the obsolete name C. bechuanicus) is a rather common terrarium species.