quick entry - Anthonis van Dyck (or his followers because of the length of the robe) a painting titled Polish Kosak. So it is perhaps not a Kosak (medium Polish cavalryman) but rather a young noble page or retainer riding his bay horse bareback, wearing a long zupan (indicating later 1640s and after), and a red 'kolpak' with 'szkofia' with three crane feather trimmed with fur.
It is located in the Bavarian State Painting Collections.
|by Anthonis van Dyck|
via Wikimedia Commons