Common Noun:
* When referring to the breed of dog: "The greyhound is known for its speed." Here, "greyhound" refers to any dog of that breed, making it a common noun.
Proper Noun:
* When referring to a specific Greyhound bus company: "I took the Greyhound to Chicago." Here, "Greyhound" refers to a specific company, making it a proper noun. It should be capitalized.
It's important to pay attention to the context to determine which meaning is intended.