Rats!
There are photos of rat pit 'fighting' where dogs are judge by how many rats they can kill in a set amount of time. I have seen photos of pencilled drawings that show the dog killing the rats as they are poured from a sack, as well as ones of rats loose in a pit.
The terriers are usually bigger, This one almost looks like a contest of "What's the smallest dog that can kill a rat?"
Is that what a Yorkshire Terrier looked like before it became a show ring dog? Almost looks more like a well built, normal face Chihuahua then a short coated yorkie.
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