A taxonomic revision of the eumenophorine tarantula genus Monocentropus Pocock, 1897, which currently comprises three species, M. balfouri Pocock, 1897 (♂♀; Socotra, Yemen), M. lambertoni Fage, 1922 (♂♀; Madagascar), and M. longimanus Pocock, 1903 (♂♀; Yemen), is presented. By integrating both morphological data and a molecular phylogeny based on mitochondrial (cox1) and nuclear (28S, 18S) markers, the genus is herein redefined to include only the type species, M. balfouri. A new genus, Satyrex Zamani & von Wirth, gen. nov., is established to comprise S. longimanus comb. nov., along with four new species from the Arabian Peninsula and the Horn of Africa described herein: S. arabicus Zamani & von Wirth, sp. nov. (♂; Saudi Arabia), S. ferox Zamani, von Wirth & Stockmann, sp. nov. (♂♀; Yemen, Oman), S. somalicus Zamani & von Wirth, sp. nov. (♂; Somaliland), and S. speciosus Zamani, von Wirth & Just, sp. nov. (♂♀; Somaliland). The new genus is partially characterised by possessing the longest male palps known in tarantulas, possibly functioning in cannibalism avoidance during mating. Both the molecular phylogeny and morphological characters suggest that M. lambertoni is probably not congeneric with M. balfouri, and also indicate that multiple species may be currently subsumed under the former name. Therefore, M. lambertoni is regarded as incerta sedis pending further studies to clarify its taxonomic placement, as it is also considered to represent a species complex. Finally, the distribution of all studied taxa is discussed within a biogeographic framework.