Scrappy-Doo is a fictional Great Dane puppy created by Hanna-Barbera Productions in 1979 as the nephew of Hanna-Barbera cartoon star Scooby-Doo. Scrappy has appeared in a number of the various incarnations of the Scooby-Doo cartoon series. Lennie Weinrib provided his voice for one season in 1979; from 1980 on, it was performed by Don Messick (who voiced Scooby).

Also, the idea of Scrappy actually may not have been all that new to the series, as he "...bore a resemblance to Spears’ and Ruby’s initial idea for a feisty little dog", which was one of the early ideas for the Scooby Doo character himself along with the "big cowardly dog" ultimately chosen.
Others considered for the voice: 
Mel Blanc was apparently the first choice, according to Evanier. This would have been interesting, considering Scrappy's connection to Henery Hawk, who was voiced by Blanc. Blanc was reportedy interested, but eventually ruled out by Joe Barbera, possibly because of concerns about his price. The second consideration was actor Frank Welker, who coined the catchphrase "Puppy Power." He would later change this to "monkey muscles" for the similar Donkey Kong Jr. character he would voice for TV's Saturday Supercade. The next choice was Messick, who was seen as giving the best audition, but still deemed "the wrong voice". Afterwards, other well known cartoon voices were considered or suggested: Daws Butler, Paul Winchell, Howard Morris, Dick Beals, and even Marilyn Schreffler. Ultimately, Weinrib was chosen, and after the first season, Messick became the final voice for the rest of the series' run.
Scrappy remained an integral part of the Scooby-Doo franchise, on both TV and in Scooby-related licensed products and merchandising, through the end of the 1980s. He was also briefly the star of his own seven-minute shorts — the Scrappy and Yabba Doo segments of The Scooby & Scrappy-Doo/Puppy Hour. Teamed with his uncle Yabba-Doo and Deputy Dusty, he helped maintain law and order in a small town in the American west.
In later years, the presence of Scrappy-Doo has often been criticized as having had a negative impact on the various Scooby-Doo series of the 1980s, though Scrappy does have his fans as well. Others credit Scooby-Doo's gradual decline during that period to other factors, such as the format changes. Scrappy-Doo has become the symbol of a character, usually overexuberant or cute in an irritating way, that critics say is gratuitously added to a series (see Cousin Oliver).
Due to the general perception of the character by audiences, Scrappy-Doo has not appeared in any Scooby-related spinoffs since the made-for-television movie Scooby-Doo and the Reluctant Werewolf in 1988, save for the first live-action Scooby-Doo theatrical film — where Scrappy played a decidedly negative role as the main villain, wanting revenge on Mystery Inc., for abandoning him years ago (he was kicked out for continuously urinating on Daphne, being obnoxious, and the final straw was when he tried to vote himself as the leader of Mystery Inc.). His plan is foiled and he and his minions are arrested in the end.
Scrappy-Doo is a recurring gag in Harvey Birdman Attorney at Law. He first appears at the end of "Shaggy Busted" to utter his catchphrase, only to be cut-off halfway when Avenger grabs Scrappy in his talons. From then on, Scrappy's corpse appears in various episodes, usually being carried around by Avenger.
He also appears in the Drawn Together episode "Lost in Parking Space, Part Two", wherein he and several other cartoon characters are brutally tortured.
He appears in the "Laff-a-Munich" skit in the Robot Chicken episode "Ban on the Fun". In the skit, Scrappy is kicked into a lake by Blue Falcon.