The letter R is sometimes referred to as the littera canina (canine letter). This phrase has Latin origins: the Latin R was trilled to sound like a growling dog. A good example of a trilling R is the Spanish word for dog, perro.
In William Shakespeare’s Romeo and Juliet, such a reference is made by Juliet’s nurse in Act 2, scene 4, when she calls the letter R “the dog’s name.” The reference is also found in Ben Jonson’s English Grammar.
Do you know other letters in the English alphabet that have a nickname besides ICAO phonetic/spelling alphabet?