That's definitely not a horn. Spidey penis or weird head leg.
The new species of
Ceratogyrus described here is remarkable. No other spider in the world possesses a similar foveal protuberance. The function of the foveal protuberance, or 'horn', in
Ceratogyrus is uncertain. Members of other theraphosid genera from the Neotropics, namely
Cyrtopholis Simon, 1892,
Sphaerobothria Karsch, 1879 and
Umbyquyra Gargiulo, Brescovit & Lucas, 2018, also possess similar foveal structures, as do some species of the ctenizid genus
Stasimopus in South Africa, and several aganippine idiopid genera from Australia (M. Rix pers. comm.).
The protuberance of C. attonitifer is unique in its length, as well as being soft, whereas this structure is fully sclerotized in all other genera where it is known to occur.
they dont know what it is
also its special because its flaccid instead of hard ( ͡° ͜ʖ ͡°)