Is a rattlesnake a proper noun?

No, "rattlesnake" is not a proper noun; it is a common noun that refers to a specific type of snake known for its distinctive rattle. Proper nouns name specific individuals, places, or organizations, such as "Timber Rattlesnake" or "Western Diamondback Rattlesnake," which would be proper nouns. In general usage, "rattlesnake" describes the species rather than identifying a unique entity.