Sparky is a stray Jack Russel Terrier and friend of Beethoven, the St. Bernard.
Beethoven was captured from a Doktor Pet Center, where he met Sparky. They escaped, but Sparky abandoned Beethoven out of fear of being captured again when the dognappers followed them. Beethoven had found himself a home with the Newton family, where Beethoven grew to a full sized Beethoven. When Beethoven would explore the town on his own, he got free food, giving some to Sparky.
Sparky ended up being captured again, and when he saw that Beethoven was now in the hands of the evil Dr. Herman Varnick, Sparky redeemed himself and paid back Beethoven by biting Varnick in his private parts.
He was confused by the appearance of Beethoven's four puppies walking along.
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Behind the scenes
In the cartoon series, Sparky was voiced by Joe Pantoliano.