I came across this picture in a book, and Lfesavr managed to find a copy of it online... not easily. I thought I'd share it and see what thoughts people have about it. Sorry we don't have a larger, clearer version.
It comes from Lydney Park, is from about 200 AD or so, and is associated with the god Nodens who seems to be a variation of Pan.
The design is similar to a Mesopotamian cylinder seal shown a short way above it on the page this came from: http://www.genesispark.com/genpark/ancient/ancient.htm
Either multiple cultures have come up with the long-neck idea out of artistic convenience, or they're basing this imagery on something.
EDIT: replaced with larger version of pic