Trouser Mice is the plural of trouser mouse, a euphemism for the human penis. This phrase began to see great use in the subject line of spam email during December 2007, offering "Safe enlargement of trouser mice". These emails advertise a substance called VPXL, an herbal enlarger.
The phrase seems to have been created out of a poor understanding of the english language; it pluralizes a singular term in a singular context. While people with supernumary penises do exist, they are unlikely to be the people targeted by this spam. "Trousers mouse" may be more appropriate, as one person may own several pairs of trousers, but is clunkier to say, and adds no more to the phrase than the singular term.