Leica released two lenses for M Mount with a variable focal length. One other did not make it past prototype stage. This article reviews the Tri-Elmar 16-18-21 and the Tri-Elmar 28-35-50 in practical use on M rangefinder cameras.
Which Leica M lens acts as a medium-angle zoom? It's the Tri-Emar, mate, and Mike is very matey with it. Here's a review from Macfilos of five years ago...