Interesting excerpt from Eques Polonus by Szymon Starowolski (1632) :
"Jazda lekka bywa dwojaka; która używa włóczni krótkiej (rohatyny) i pancerza, nazywa się Czeremysy i druga, która albo strzelby tylko, albo łuków, albo obojga razem używa, bądź zbroję odziana bądź nie, nazywają się kozacy [...]
Czeremysy w pancerze i tarcze zbrojni, albo strzelby długiej, albo strzał, opasani szablą krzywą, mają oni włócznie na 4 łokcie długą, z boku prawego uwiązaną, którą przy spotkaniu z bliska rażą mocno nieprzyjaciela. Kozacy są dwojacy: jedni odziani w zbroję albo pancerz i szyszak, i mają dwie strzelby krótkie, i jedną długą, i szablę krzywą. Drudzy kozacy bez zbroi, w samych tylko sukniach, używają tejże strzelby"
"There are two types of light cavalry. One is equipped with spear and wears chainmail - this one is called czeremysy and the other one is equipped with long firearm or bow or both and is furnished with armor or not - this one is called kozacy [...]
The czeremysy are furnished with chainmail and shield, equipped with long firearm or bow, sabre, 2,5 meter long spear tied to the right side, which they use to attack their enemies at a close distance.
There are two type of cossacks: first type is furnished with armor or chainmail and szyszak helmet and is equipped with two short firearms and one long as well as sabre. The other type is unarmored, only in their coats equipped with long firearm"
As you can see the description is quite a mess :p. The terminology was not standarized as well as the equipment. Light cavalry was sometimes called - cossack, tatar, wallachian, elear.
As for the czeremysy this seems to be just another name for petyhorcy. Which in turn at some point used shorter version of hussars's kopia.
As for the shields - we do not know for sure what kind of them were in use - this could be round eastern woven one, balkan asymmetrical or something else.
Note on the translation:
I've used long and short firearm for długa and krótka strzelba because it doesn't specify whether its a carabine or arquebus and pistol or krucica (sawn off arquebus)
As for armor and chainmail - in period texts zbroja (armor) designated some kind of plate armor and pancerz - chainmail.