Bow-Wow may look like your average terrier. The streets he walks may seem familiar. But just around the corner, things get a little unusual.      With nary a word, Mark Newgarden and Megan Montague Cash have created a story about a bold new doggy who goes where no doggy has gone before. With a spring in his step and his tail only occasionally between his legs, Bow-Wow faces down every foe--well, almost every foe--in his path.Step aside, mutts. There's a new dog in town. (20070501)