...the only God, who is at the Father's side, he has made him known.
John 1:18 ESV

...the only begotten Son, which is in the bosom of the Father, he hath declared him.
John 1:18 KJV

There's a big difference in apparent meaning between "the only God" and "the only begotten Son". The translations differ because ancient manuscripts differ. The oldest available manuscripts say "God". Most manuscripts (centuries later) say "Son".

Fortunately, it only matters if you take this one verse out of its context. Neither rendering changes the overall meaning of Scripture.