=MP ramag (s S. Shaked, JAOS 1993 78f.); for fem. gender see Bh Gr2 10(25).
In Rabbinic tradition this kind of mule is offspring of a female horse and male ass, and refuses to be bridled and is thus particularly swift. See discussion in Levy.
We assume that using this image for "a multitude" of people has a negative connotation similar to what it would in English.