1627: R: a Dog's Letter May 29, 2019

Much of the way that people know how languages sounded in the past is simple analogous descriptions. For example, writers in both Latin and Middle English would liken the pronunciation of R to the growling of dogs, indicating that the sound was trilled, unlike in Modern Standard English. In fact, a name for the letter R in Latin was 'littera canīna' which is literally 'the dog's letter'. This might sound silly, but these sorts of things have helped to further linguistics.
For more about R, read this.