According to my Analytical Greek Lexicon by Friberg, Friberg and Miller:
" monogenes - of what is the only one of its class, unique (1) an only child born to human parents [one and only Luke 7:12; 8:42]; substantivally only child (Luke 9:38) (2) as a child born in a unique way; (a) used of God's Son Jesus only, only begotten; substantivally (John 1:14) (b) used of Abraham's son Isaac only; substantivally his only true son (Hebrews 11:17)"
As a former Evangelical, I know that there is a myth among many about the Koine Greek of the NT. The myth is that the Greek is very precise, much more precise than English.
When I started studying Theology at an Evangelical College, I decided I was going to study Greek for 3 years so I could understand the NT precisely. What I learned, is that scholars of Greek, disagree over many words. The preciseness belief I had fallen for, was a myth.
From the above, from the Analytical Lexicon, the term monogenes (only begotten) is used for slightly different meanings, in different places, just as English words can have variant meanings.
As for the posters argument that the Creed is wrong because 'begotten' makes Jesus subordinate to the Father, well, IMO that is missing the point entirely. The fact that Jesus is Son makes Him subordinate to the Father.
There is no other Son who is uniquely begotten, because the begetting is in Eternity, where there is no time. No one can explain how the Son is begotten, it is a mystery.