"Horde" is a derogatory form of "group", or "pack".
Used to describe large numbers of usually unsavoury, unwholesome, or unwanted animals or people.
e.g. A horde of rebels/looters/vermin etc
There is no such thing as a "pack rat." It is a sterio type given to rats because of there burrowing nature. but roof rats and norways can mate.
Pack rats do, indeed, exist (see link) but they are...