Etymology, dictionaries, grammars
pertaining to plural formation:

one cowboy, too many cowboys,
one cowgirl, not enough cowgirls

one lone horseman riding into sunset,
four horsemen summoning apocalypse

one ox calmly yielding to two oxen
while sheep stubbornly remain sheep

mice not mouses, lice not louses,
but one goose begets more geese

and in place of meese or mooses
roam the remnants of the moose

just one last prickly conundrum:
is it cactus, cacti, or cactusses?