A 1906 issue of Scientific American examines motorized roller skates exhibited at that year's Paris Automobile Show. There were several models in development by different inventors at the time, including these by French inventor M. Constantini. They're basically tiny cars that he wore on his feet:
In view of the fact that each skate contains a gasoline motor, carbureter, battery, and spark coil, it will be seen that the whole has been reduced to a comparatively small size. The use of the rubber-tired wheels is found to give a very smooth-running movement. On the back of each skate will be observed the small sheet-iron box which contains the battery and the spark coil. From the box a pair of wires protected by rubber tubing passes up to the leather belt which the person wears, 'and upon the belt is placed the switch by which he is able to make or break the ignition circuit when he wishes to start or stop the motor or to regulate its speed. On the back part of the belt is fixed a small gasoline tank in the form of a flat and slightly curved sheet-iron box.
You can see patent drawings of other motorized skates at The Old Motor, a fun website about strange antique cars.