52. Greek nouns of the 2nd declension end in -os, -ōs, masculine or feminine, and in -on neuter.
They are mostly proper names and are declined as follows in the singular, the plural, when found, being regular.
a. Many names in -ēs belonging to the 3rd declension have also a genitive in -ī.
Thūcȳdidēs, Thūcȳdidī (compare § 44.b).
b. Several names in -er have also a nominative in -us.
Teucer or Teucrus
The name Panthūs has the vocative Panthū (§ 81.3).
c. The genitive plural of certain titles of books takes the Greek termination -ōn.
Geōrgicōn of the Georgics
d. The termination -oe (for Greek -οι) is sometimes found in the nominative plural.
Adelphoe the Adelphi (a play of Terence)
e. Greek names in -eus (like Orpheus) have forms of the 2nd and 3rd declensions (see § 82).