User blog comment:Aleksandr the Great/Prothean Squadmate is Officially DLC/@comment-4865682-20120222150658/@comment-1221773-20120223004644

"Prothean" is supposed to be a term created by modern races to refer to the previous galactic civilization, hence why it's a capitalized proper noun and other race names (asari, human, batarian) are lowercased common nouns. We're not sure if the Protheans actually called themselves "Protheans".