I have the "Spunkless Spunky" episode on and I just now noticed something. In Spunky's first episode the dog was a female. If you listen to Fonzie describe the dog thats a clue. Then the second time he is a male! Also they used a different dog the second time. The first dog was slightly smaller. And it was a slightly darker color!

Even within just the "Spunkless Spunky" episode, more than one dog was used. The first one appeared to be whiter and a little bigger.