The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Greek Nouns of the Third Declension.
81. Many nouns originally Greek - mostly proper
names - retain Greek forms of inflection. So especially -
1. Genitive singular in -os, as,
2. Accusative singular in -a, as,
3. Vocative singular like the stem, as, Periclé, Orpheu, Atlá.
4. Nominative plural in -es, as,
5. Accusative plural in -as, as,