thanks finds.org.uk DEN-0F2C87 CC BY 2.0
I found it on the Portable Antiquities Scheme website, a project funded by the UK government to "promote the voluntary recording of archaeological objects found by members of the public in England and Wales." You should go look at it. It's cool.
Animal imagery played a huge part in medieval art, not least for its symbolism; I can well imagine a protective padlock would be made in the shape of a guard dog. Here is an essay at the Met website on the subject.