An agental suffix derived from ancient ✶-rō̆ (WJ/371), usually appearing as suffixal -r but sometimes as -ro as in quentaro “speaker, minstrel” (PE19/83; PE18/51). The fuller form appears especially in cases where the word is taken to be masculine: compare its feminine form [ᴹQ.] quentare “minstrel [f.]” (PE23/85). Tolkien indicated -rō was sometimes accompanied by nasal infixion, and that there was another variant -rdŏ > rd (WJ/371), but there are no clear signs of -r(d) in his later writings (but see the ᴱQ. examples below).
Based on the example ontari “parents” plural of [ᴹQ.] ontaro “parent [m.]”, it seems the usual plural form of this suffix may be -ri even in cases where -ro survived into Modern Quenya. This plural could be because of Quenya’s general dislike of repeated r-sound. However, ontaro may also be a specifically masculine form of an unattested gender neutral ✱ontar “parent” and ontari may be the plural of that gender neutral form, in which case likely there is no plural for the specifically masculine form. Regardless of the exact phenomenon in play, I would assume the plural of words like quentaro “minstrel” is quentari, referring to a group of minstrels of any gender.
Conceptual Development: There are no examples of -ro in the Early Qenya of the 1910s and 20s, but there are examples of agental -r and sometimes -ar, as in ᴱQ. maksar “cook” from ᴱQ. maksa- (QL/59), and ᴱQ. vaktelear “merchant” from ᴱQ. vaktele “trade” (QL/99). There are also examples of -r(d), such as ᴱQ. ektar (ektard-) “swordman” (QL/35) and ᴱQ. moar (moard-) “shepherd” (QL/60).
A prefix having to do with rising, sunrise and the east, appearing as ro-, hró-, and orró- and derived from √(S)RŌ (UT/165; PE17/18). Compare with the prefix oro- “up, aloft” of similar derivation (PE17/64).