Although Shaggy is referred to as a sheep dog, the dog who played Shaggy (Cole) is a Bearded Collie, a herding breed known for their intelligence, energy, enthusiasm, and ability to jump (known as "the Beardie bounce").
The trivia item below may give away important plot points.
During the scene where Dave (as a dog) flies, he says "To infinity and beyond!" before landing on the bus to the courthouse. This is a reference to Buzz Lightyear in the Toy Story franchise, who was also played by Tim Allen.