XDog (2013~2016), as far as we know, should be the first four-legged robot driven by disc-type out-rotor brushless motor (8108 Motor) in the world. Check XDog videos.
In the early 2014, the knee joint of XDog was tested driven by synchronous belt. Check belt-driven video. But it was changed to link transmission afterwards.
8108 outrunner brushless motor have many manufacturers.
For more details, please check our Youtube channel.