Of what ethnicity were the Cossacks?

The first Cossack companies were formed in the 15th cent., when Ukraine, then part of the unified Polish-Lithuanian state, took independent measures to defend itself against the devastating Tatar raids. The Ukrainian Cossacks, of heterogeneous background, were chiefly Russians and Poles and included many runaway serfs. By the 16th cent. they had settled along the lower and middle Dnieper River (for their history to 1775, see Zaporizhzhya). Similar communities grew up on the Don (see Don Cossacks) and its tributaries. They were all organized on principles of political and social equality, and originally were virtually autonomous.

Cossacks: A Slavic warrior caste known throughout Russia, Ukraine and Northern Mongolia. The name Cossack probably originates from Turkic, "Kazakh" meaning either "horseman" or "free man" (i.e. not a serf or noble) depending on context. Both definitions hold true, as Cossack warriors were exclusively cavalry, and actively recruited freed or runaway serfs into their ranks. Going by the Turkic/Mongolian origin of their name, the Cossacks may have originated in Central Asia, and migrated into the Slavic lands as nomads, perhaps on the heels of the Mongol invasions.

Historically, the Cossacks were predominantly Russian Orthodox Christian, but there were a few, especially around Crimea, who were Muslim, and some were even Buddhists from Mongolia.