Linguist Hrachya Acharyan was of the opinion that the graphic form of letter Ռ had been derived from its Iranian prototype, while according to the opinion of German orientalist Joseph Markwart, it had been formed by combining two Armenian letters Ր [soft “r”]. The second version is corroborated by the fact that phonetically Ռ is a variant of Ր, pronounced with bigger vibration. Native speakers of other languages might find it strange that there are two letters “r” in the Armenian language.
Even our compatriots living in Diaspora often fail to feel the difference between these two letters. However, substitution of one sound with the other might distort not only the pronunciation, but also the meaning of a word, since there are numerous words in the Armenian language, in which Ռ and Ր play a distinctive role. While the word կռունկ [krrunk], for example, means “crane”, the word կրունկ [krunk] stands for “heel”; դեռ [derr] – “yet”, while դեր [der] means “role”. Or let’s take the word լուռ [lurr] – “silent”, which, with the soft “r” is transformed into “news” – լուր [lur], etc. The numerical value of Ռ is 1000.
This letter signifies the beginning of the thousands’ row: after it, the value of each letter successively increased by one thousand. It is interesting that while the Armenian Ռ opens the row of thousands, the Latin R is less lucky. The ancient Roman writers called it “littera canina” – the dog's letter. The nurse in William Shakespeare’s “Romeo and Juliet” also calls it the dog’s letter: What a mocker you are! It is R-r-r – the dog's letter!” The rolling sound of R reminded the ancient Roman writers and later – the great English poet, about the growl of a dog.
See more at Aram Khachaturians "Armenian Alphabet. History and Symbolism" book