pecus

Pecus is two nouns sharing one nominative, and the corpus has both. Pecus, pecoris, n., third declension, is a herd or flock taken as a mass, so pecora are herds. Pecus, pecudis, f., is a single head of livestock, one beast, so pecudes are the individual animals. Genitive singular is the only thing separating them, and a translator who has not noticed the split will be baffled by seeing pecora and pecudes in the same paragraph.

The root reaches further than the farmyard. Pecunia, money, is what you have when your wealth walks about on four legs, and peculium, the private fund a slave or a son was allowed to build up, comes from the same place. So does peculiar, by way of peculiaris, "belonging to one's own property".