In Greek mono by itself means "only" so monogenes has to mean something more. As the earliest fathers were writing in Greek, they would have no need to define or translate the term.
Justin Martyr (circa 150 AD) says in his First Apology, chapter 23:
Jesus Christ is the only proper Son who has been begotten by God, being His Word and first-begotten, and power
This demonstrates to me that he saw the begotten aspect as part of the term monogenes.
Tertullian (circa 210) says in Against Praxeas, chapter 7:
"The Lord created or formed me as the beginning of His ways;" then afterward begotten, to carry all into effect--"When He prepared the heaven, I was present with Him." Thus does He make Him equal to Him: for by proceeding from Himself He became His first-begotten Son, because begotten before all things; and His only-begotten also, because alone begotten of God, in a way peculiar to Himself, from the womb of His own heart-even as the Father Himself testifies: "My heart," says He, "hath emitted my most excellent Word."
Also, in the Arian controversy we clearly see they included the concept of begotten in their interpretation rather than deleting it as modern translators tend to do. Otherwise, the Nicene Creed would not say "γεννηθέντα, οὐ ποιηθέντα." That is, "begotten, not made."
I found an interesting article, An Inductive Study of the Use of Monogenes in the New Testament written by a Doug Kutilek, arguing that the real meaning is simply "unique" or "one of a kind." However, he admits that this is clearly not how it was interpreted in the third and fourth centuries. His last paragraph:
Understanding monogenes in its proper sense--one that completely excludes any notion of “begetting” or “begotten”--has strong theological implications for the doctrine of Christ. It renders moot the whole heated theological debate of the third and fourth centuries concerning the so-called “eternal generation of the Son,” a term which always left me with the uncomfortable feeling that if we accepted such terminology at face value, we were admitting de facto that Christ was a created being and not God. It also makes the Nicene Creed’s affirmation that Christ was “begotten but not made” (gennethenta, ou poiethenta) so much verbal nonsense.  Likewise, proposed translations of monogenes such as that noted in Arnt and Gingrich’s Greek Lexicon, namely “begotten of the only one” are exposed as wholly ludicrous and unfounded.  Christ is the unique Son of God; that is, in the sense in which He is the Son of God, He has no brothers.