...I stumbled upon a a few others.
|The Cure of Folly by Hieronymus Bosch (c1475-80) - Mule Detail|
|Die Eidesleistun by Derick Baegert (c1493-94)|
|Geißelung Christi by Albrecht Dürer (c1494-95) - Mule Detail|
|Pilatus wäscht seine Hände in Unschuld by Albrecht Dürer (1512) - Mule Detail|
|Christus vor Kaiphas by Albrecht Dürer (1512) - Mule Detail|
|Saint Jerome in his Study by Albrecht Dürer (1514)|
|Dance of Death: XX The Magistrate by Hans Holbein the Younger (1538)|
There is another representation near the bottom of this page, but it doesn't give dates, which is very unfortunate. I've been trying to track it down, but to no avail.
According to Dictionary.com, a mule, or a lounging slipper, has its origin from 1350–1400 Middle English, which perhaps came from the Middle Dutch for mūle. Some sites claim that a mule was a precursor to clogs. Clogs appear to have been worn outside the home in a work environment. These representations appear to be for use within the home, a bath house, or for comfort's sake, though one of them is being worn by a soldier while persecuting Christ. Later, and just outside our period (1600-1625), is the following British find in the Met Museum: