Hi Fusilier!
First example of Chevauxlegers being called Cossacks that comes to my mind, without rechecking, is the "O'Reilly-Guard Cossacks" in the Austerlitz OoB...they are ment to be Chevaux-legers...(the ones that tried to cover the allied retreat with multiple, sacrifical charges...apart from Rosenberg- and Vincent Chevauxlegers - which I haven't checked - will do so tomorrow and report back...)
They are a part of Kienmayers Avant-Garde...
CvC