hostis

Hostis, hostis is third declension, masculine and feminine, an i-stem, so the genitive plural is hostium. Latin normally uses the plural hostes for the enemy as a body, where English uses a singular collective, and the singular for one enemy individual.

The etymology is the good bit. The oldest sense is "stranger", and before that "guest": hostis and hospes are the same root, which is how one word feeds English both hostile and hospitality. The distinction that matters in reading is against inimicus. Hostis is the public enemy, the state you are at war with, and declaring a citizen hostis was a formal act with legal consequences, not name-calling. Inimicus is the man you personally dislike. Family: hostilis, hostilitas. English hostile, host in its older sense of an army, and Spanish hueste.