Here's why:
* It names a group of people: Cossacks are a historical and cultural group of people known for their equestrian skills and military prowess.
* It can be the subject of a sentence: "The Cossacks were renowned horsemen."
* It can be the object of a verb: "We learned about the history of the Cossacks."
Let me know if you have any other words you'd like to identify the part of speech for!