The literal definition of μονογενης as "the only one of its kind" (or "only") , based on its morphology, was most probablyis highly likely the original due to being attested in pre-Christian Hebrew and Greek manuscripts of the Old Testament.
(cont.) It μονογενης as "only" is also the only consistent definition found in early biblical manuscripts in all languages (Greek, Hebrew, Syriac, Coptic, Latin) in the early church (circa 2nd-4th century CE).
The figurative meaning "beloved" of μονογενης and its synonym αγαπητος was attested numerous timesfrequently in the Septuagint. Both μονογενης and αγαπητος translates the same Hebrew yachid (Literally, "only").
The definition 'only begotten' was already present as early as the second century CE. It was the universally accepted meaning by the church fathers both in the east and the west in regards to its application to Jesus Christ even before the Arian controversy.