* If you're referring to the mythical creature, then Bigfoot is a proper noun. It's a specific name for a specific being.
* If you're referring to a large footprint, then big foot would be a common noun. You're not talking about a specific footprint, but the general concept.
Here are some examples:
* Proper Noun: "Have you ever seen Bigfoot?"
* Common Noun: "That's a big foot! It must have been made by a giant."