The Siberians help to make up the heavy Russo-Mongolian troops. They are tough soldiers who learned the art of war in Siberian boot camps, known for their extremely high draftee mortality rates. Soldiers who are lucky enough to make it out of these camps alive are forever marked by a fierce hatred towards their fellow man, which makes them terrifying enemies.
It is odd that Siberians would wear a Budenovka in the case of it being a symbol of the Red Army, a foe of Ugenberg. Maybe it is worn with the combination of a gas mask to look intimidating.
On the US retail box art, a Siberian is on the front.