This is a cave canem, or, a "beware of the dog" sign. A dog skeleton was also found inside the Thermopolium.

Thanks! In the close up it's much more clearly a dog.

There seems to be some scribbling on the top black lining, looks like "NICIλ something something". Any idea on what it says?

Also, I'm surprised by how similar the collar design looks like a typical encrusted dog collar you could find at any dog park today.