Lisowczyks - Polish-Lithuanian 'riders of the Apocalypse'

 Lisowczyks - Wikipedia

The Polish Rider by Rembrandt. A Lisowczyk may be the subject of one of the Dutch master's greatest works. Though the rider's identity is not known, one theory is that it is a portrait of Grand Chancellor of Lithuania Marcjan Aleksander Oginski, made in c.1655. It has little to do with the Lisowczycy, though much of the clothing and war gear would have been similar that worn by the real Lisowczyks of 30 years earlier.

Michal Paradowski who knows a lot about this subject says 'Let's just say of 'Polish-Lithuanian Commonwealth' origin, as it was total mix of different mercenaries. While they were serving in Imperial armies, they were just called 'Cossacks' although it should be really 'cossacks' as in cossack cavalry (jazda kozacka) not as 'Cossacks' (i.e. Zaporozhian Cossacks). Fairly short-lived formation. I wrote a wee bit about them in my books, they will be mentioned again in my new one that, hopefully, will be published later this year.'