ἐριαύχενας ἵππους
Parry notes that ἐρι- is the Aeolic prefix. The Ionic form, ἀρι-, has the same metrical value, and so we would expect such words to shift to the Ionic. But, Parry argues, exceptions to that general rule of the evolution of Homeric language, such as we have in this case, show that “not only the bards’ habit of using epithets with the Aeolic prefix, but also their sense of the formula which contained such epithets, protected the prefix here from their tendency to keep only those elements of the λέξις ξενική which differed metrically from the corresponding Ionic forms.” Parry also notes that ἐριαύχενας appears only within this formula (Parry, MHV 181n1). From this we can see how conservative the oral composition process can be, in addition to how formulaic phrases can be thought of, used, and preserved by the singers as a unit.