Possible Clues:

• Maybe sheep or deer drained pool and river?
• 'Men' or 'teeth,' grammatically
• "Foci" or "fungi"
• Certain nouns and verbs
• What 'they' can only be, to grammar sticklers
• Like words ending in S
• Odds or evens, e.g
• Like ghosts and goblins?
• More than one comrade touring Lima, an old city
• "Octopuses" or "octopi"