Jaffee

Jaffees are blood-sucking, Leech-like parasites that are able to take the form of their host. They were native to planet Melmac, prior to its destruction. Not much is known about Jaffees, except that they overran the city of Detroit and renamed it Jaffeetown. It is also known that Jaffees are not vulnerable to boric acid, as Earth bugs are.

Behind the scenes
Jaffees have never been actually seen, they were mentioned in one episode of ALF. The name "Jaffee" actually comes from the last name of one of the workers on the show.