A couple of my friends have horrible mothers in law. Both of said friends are the nicest people you'd ever meet. A couple of work friends came up with the moniker, TAG (troll at the gate) to refer to these harridans. I prefer ETAG (evil troll at the gate).
Naming nasty people is so cathartic. At right, Viola Swamp from the children's book Miss Nelson Is Missing. The epitome of harridan.