But PACAPAINS are an exception:
Pīrāta, agricola, collēga, aurīga,
Poēta, āthlēta, incola, nauta, scrība.
But HARP and trees are exceptions:
Humus, Aegyptus, Rhodus, papyrus,
Quercus, laurus, alnus, and cedrus.*
-S, -Ō, -X: Feminine,
-L, -A, -N, -C, -E, -T: Neuter.
Sol (m), piscis (m), leō (m), mōns (m), are exceptions.**
But manus is an exception;
-ū (cornū) is neuter,
As are genū, gelū, and pecū.
Like rēs, faciēs, and glaciēs,
But diēs is an exception.
It's mostly masculine,
But sometimes feminine.
*The Second Declension contains -us masculine nouns (like equus), rare -us feminine nouns (like papyrus), and many -um neuter nouns (like verbum), for which there are no exceptions of which I'm aware.
**Third Declension exceptions abound. Here are some general rules with some common exceptions.
Masculine Rule: -ōs, -ex, -es, -er, -or (Oh, sexes! Error!) (Exceptions: iter n, ūber n, acer n, vēr n... arbor f, aequor n, marmor n, cor n).
Feminine: -s (preceded by ā, ē, i, ū and consonants... not by ō... see masculine rule), -ō, -x (except when preceded by -e... see masculine rule) (exceptions: leō m, sermō m, ōrdō m... ignis m, sanguis m, collis m, mēnsis m, orbis m, piscis m, oriēns m, mōns m, pōns m, fōns m, dēns m..., pēs m, vās n.)
Neuter: -l, -a, -n, -c, -e, -t, -us (-ris), ōs (-ris), -ar, -ur (exceptions: sōl m, sāl m, lepus m, pecus f).