Lewis & Short

Y, y, a Greek letter introduced at a late period for words borrowed from the Greek, the place of the Greek Υ being previously filled by U (i. e. V, which graphically originated from Υ; v. the letters U and V). Thus, according to the express testimony of Cicero (Or. 48, 160), Ennius always wrote Burrus for Pyrrhus, and Bruges for Phryges; and so the words which were identical in Greek and Latin in the oldest period of the language have either preserved u where the Greek has υ, as bucina and βυκάνη, cubus and κύβος, fuga and φυγή, mus and μῦς et saep.; or this u has given place to i, as in lacrima, formerly lacruma, = δάκρυμα. Sometimes, also, o took the place of the υ; cf. mola and μύλη, sorex and ὕραξ, folium and φύλλον, and, shortening a long vowel, ancŏra and ἄγκυρα, like lacrĭma and δάκρῦμα. In Cicero’s time y seems to have been already in use; but its application was restricted to foreign words, and hence the spellings Sylla, Tybris, pyrum, satyra, etc., are to be rejected.