The dog in Zeus and Roxanne is a Portuguese Podengo, not a mutt.
(they actually used siblings)
They use podengos often to portray 'mutt' type dogs. As well as the obvious Picardy used in Winn Dixie. It's not too bad because no one realizes these dogs are purebreds.
They use them because most dogs in movies are played by multiple dogs. It's hard to find multiple mutts that look alike enough to use in a movie as the same animal.
Winn Dixie used 4 Picardies.