What does Valachus mean in Latin?

Valachus, a noun, means “Vlach”.

What is the genitive of Valachus?

The genitive singular is Valachī. Valachus is masculine, second declension.

noun · masculinesecond declension

Valachus m (genitive Valachī)

masculineVlach
Altera Europæa Sauromatia dicitur: cuius coloni ſunt Poloni, Ruſſi ſiue Ruteni, Lituani, Maſouitę, Prutenni, Pomerani, Liuones, Moſchouitę, Goti, Alani, Valachi, & Tartari qui in hac ripa Tanais fluuij occidentali, iuxta Pontum Euxinum degunt.The other is called European Sauromatia, whose inhabitants are Poles, Russians or Ruthenians, Lithuanians, Mazovians, Prussians, Pomeranians, Livonians, Muscovites, Goths, Alans, Vlachs, and the Tartars who live on this western bank of the Don river, near the Black Sea.1581, Alexander Guagnini, Sarmatiae Europeae descriptio, quae regnum Poloniae, Lituaniam, Samogitiam, Russiam, Massoviam, Prussiam, Pomeraniam, Livoniam, et Moschoviae, Tartariaeque partem complectitur [Description of European Sarmatia, Which Encompasses the Kingdom of Poland, Lithuania, Samogitia, Russia, Mazovia, Prussia, Pomerania, Livonia, Muscovy, and Part of Tartary], folio 1r: