I was reading a book called Bigfoot vs. Chupacabra in which this issue was raised. I suspect it derives from Tolkien, per the Proudfoots vs. Proudfeet dispute.
- What is the proper plural of the colloquial American term for Sasquatch?
The book also touches on Yeti, which seems to be both singular and plural, although "Yetis" is proposed as a correct, if less graceful, alternative. I mention it because there may be a plural usage for Bigfoot: "Stay away from the crick--there's a herd of Bigfoot down there eatin' blueberry bagels."
As I am regularly accused of assaulting the English language, I though I'd bring it to the experts.
This question on "mouses vs. mice" provides some very good insight, but I'm also interested in these specific terms, and the idea of the plural use of the singular form, thus Yeti/Yeti/Yetis. (In other words, it also relates to deer and deer (singular and plural, respectively.)