What does Monogenēs mean in Latin?

Monogenēs, a proper noun, means “an Aeon of the Decad, forming a syzygy with Macaria (in translation, glossed “Only-Begotten”)”.

What is the genitive of Monogenēs?

The genitive singular is Monogenis. Monogenēs is masculine, third declension.

In classical Latin, [mɔˈnɔ.ɡɛ.neːs]; in ecclesiastical (church) Latin, [moˈnɔː.d͡ʒe.nes].

proper noun · masculinethird declension

Monogenēs m sg (genitive Monogenis)

Classical[mɔˈnɔ.ɡɛ.neːs]Ecclesiastical[moˈnɔː.d͡ʒe.nes]

Late Latinan Aeon of the Decad, forming a syzygy with Macaria (in translation, glossed “Only-Begotten”)
AD 207–211, Quintus Septimius Florens Tertullianus (author), Mark Timothy Riley (editor, translator), Adversus Valentinianos in Q. S. Fl. Tertulliani adversus Valentinianos: Text, translation, and commentary (PhD dissertation, February 1971), Text, chapter vii, page 33Consequently, he himself is called "Father" and "Beginning of the universe" and (as his proper name) "Monogenes" (i.e., only begotten), though not properly since he is not the only legitimate child. Another was born at the same time, a female whose name is Truth and for that reason, Monogenes might have been named Protogenes much more aptly as being the first born. — ibidem (Translation), page 83
ibidem, chapter viii, pages 34–35I will enter the names of the ten I mentioned: Bythios (Profound) and Mixis (Mixture), Ageratos (Unaging) and Henosis (Union), Autophyes (Essential Nature) and Hedone (Pleasure), Acinetos (Unmoved) and Syncrasis (Commixture), Monogenes (Only-Begotten) and Macaria (Happiness). — ibidem, page 85

mŏnŏgĕnēs, is, m. μονογενής,

I. onlybegotten, only, Tert. adv. Valent. 7 sqq.

Substantivised epithet from the Ancient Greek μονογενής (monogenḗs, “unique of its kind”).