A group of cats is known as a glaring, a clowder, or a clutter of cats. Glaring may have a sort of shaded meaning in that it is sometimes defined as a group of cats who don’t exactly get along with each other or who live in some sort of a tenuous peace with each other.
Glaring at least is from 1450, Middle English. Exact etymology is not known, but it may be a Gaelic borrowing.
Clowder is related to clutter. Clutter is a Welsh borrowing. Both are from around the same time period as glaring.
There used to be all sorts of collective nouns for many common species, but most of these words have dropped out of the language. A very interesting one is a murder or crows -> a group or flock of crows. We still have herd for cows, pack for dogs, school for fish and flock for birds, but most of the rest are gone.
