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.