Other words following the same usage are, pair, brace, dozen, after numerals (if not after numerals, or if preceded by the prepositions in, by, etc, they add -s): also trout, salmon; head, sail; cannon; heathen, folk, people.
The words horse and foot, when they mean soldiery, retain the same form for plural meaning; as,—
The foot are fourscore thousand,
The horse are thousands ten.
—Macaulay.
Lee marched over the mountain wall,—
Over the mountains winding down,
Horse and foot, into Frederick town.
—Whittier.
III. Plurals formed by Adding -s or -es.
42. Instead of -s, the ending -es is added—
(1) If a word ends in a letter which cannot add -s and be pronounced. Such are box, cross, ditch, glass, lens, quartz, etc.
-Es added in certain cases.
If the word ends in a sound which cannot add -s, a new syllable is made; as, niche—niches, race—races, house—houses, prize—prizes, chaise—chaises, etc.
-Es is also added to a few words ending in -o, though this sound combines readily with -s, and does not make an extra syllable: cargo—cargoes, negro—negroes, hero—heroes, volcano—volcanoes, etc.