The Sussex spaniels is one of the older breeds of spaniel. The modern dog has short-legs and a long back, although it did not always have those characteristics. In fact, look at this depiction of the breed by George Stubbs:
The dog at the horse’s feet is listed as “Springer spaniel,” which did come in wider array of colors than the modern breed.
However, the Sussex dog to the right looks like a much larger spaniel than what we call a Sussex spaniel today. It is rapidly become a Museum piece dog, along with the Clumber spaniel, the other heavily set spaniel.
Today’s dog looks like Tootsie Roll, and it is almost extinct. No one wants a short-legged spaniel, even if it can charge through the cover a little better.