The word manes is Latin and in English is pronounced MAY-neez or perhaps MAH-nays. In English the word has no singular, and so NetHack just uses manes for the singular as well. In Latin the word had a singular, manis, but this was apparently not used to refer to spirits of the dead (the NetHack sense), only to other meanings of the word. Presumably Romans used other words to refer to a single spirit. Compare lemure.
It is an exceptionally unfortunate character who has trouble defeating manes; however, swarms of them can overwhelm weaker characters. Getting into a corridor or doorway where they can attack only one at a time is an effective tactic.
In Roman mythology, manes are the spirits of the dead. Wikipedia has more details.
The gnats of the dungeon, these swarming monsters are rarely