Kos, meaning "blackbird", is a Slavic surname.
It is very common in Slovenia, Croatia, Poland, Ukraine, and the Czech Republic.
It is the 10th most common surname in Slovenia.
[1] Unlike many other Slovene surnames, Kos is found throughout the country, although it's more common in the central and eastern regions than in the west.
[1] It is the third most common surname in the Zagreb County of Croatia.