Vox Canina

Vōx Canīna (canine voice) or Littera Canīna (canine letter) was the name used by Romans to identify their pronunciation (as an alveolar trill) of the Latin letter "r" (Greek "rho"), as well as the letter itself. The reason for this terminology was that the sound of a rolled, voiced, non-initial "r" was thought by the Romans to be similar in sound to a dog's growl.