I was reading "Fantastic Beasts and Where to find them", and when I read the entry on the Jarvey, I realised that there is actual evidence to support their existance right here in Britain! Maybe JK Rowling was inspired to create them as a way of explaining this extraordinary story...
The Jarvey is said to be able to talk, although mainly limiting itself to rude word or phrases, and looks like 'an overgrown ferret'.
In 1931 on the Isle of Man in the UK, the Irving family were allegedly host to a talking mongoose, who called himself Gef. His choice of words was mostly limited to swearing and joking! He was apparently shot 10 years later by a famer who bought the land where he lived.
Wikipedia have a great article on 'Gef the talking Mongoose' if you're interested!
Petronella_Pettigrew
PS This may be clutching at straws, but the paranormal investigator who visited the Irvings to reseach Gef was one Harry P...
(Harry 'Price', sadly, but who knows...?)
. Maybe JKR based Harry Potter on this Price fellow, or just got the name from him. Farther reseach is in order...
Oops. I'm Godric.