From Textus Receptus
Monogenēs (Unigenitus) is a Greek term (μονογενής) in Christology, usually translated as "Only begotten". This is based on deriving monogenēs from monos μονος (only) and gennaō γενναω (beget). A false definition derives monogenēs from genos γενος (kind or type, cf. Latin genus), so means 'unique', 'one of a kind'. This is falsly claimed to be supported by New Testament usage: Hebrews 11:17 refers to Isaac as Abraham’s ‘only begotten son’, yet Abraham begat other sons (Ishmael, and by Keturah). But Isaac was Abraham’s unique son, the son especially promised to him and Sarah in their old age, and who would carry on the covenant line.
Strangly the Wescott and Hort text actually reads "Only begotten God".
Dean Burgon stated - “We are offended at reading (against S. John 1:18) – ‘Many very ancient authorities read God only begotten:’ whereas the ‘authorities’ alluded to read ‘monogenes Theos’ – whether with or without the definite article prefixed – which, as the Revisionists are perfectly well aware, means ‘the only-begotten God,’ and no other thing. Why then did they not say so? Because, we answer, they were ashamed of the expression.” Ibid, pg. 182.