Time was, a man could play a guitar in Times Square in a cowboy hat and tightie whities and be assured that he was the only one doing it. That’s what Robert Burck, who has made a living over the past eight or so years as the Naked Cowboy, thought, anyway. “I am the epitome of the American dream,” he once said. “I’m Captain America, but better.” But recently, his dream has been threatened: by an M&M. Mars, the company that owns M&Ms, recently erected a billboard in Times Square on which a blue M&M in underwear and a cowboy hat strums the guitar, much like Burck. “My initial response was like, ‘Wow that’s cool,’” Burck told the Post the other day. But then he thought twice: He had spent years becoming the Naked Cowboy, honing his act, perfecting it. And now a piece of chocolate waltzes in and assumes his identity? Burck was not having it. This past Monday, he filed a $6 million suit against Mars for trademark infringement and other claims. The threat of the lawsuit seems to have worked; the Post today reports that Mars has taken down the billboard with the offending caricature. But this is not just a victory for the Naked Cowboy, it’s a victory for us all. “I represent the little guy,” he said.