Michael Bloomberg was due to arrive any minute at the downtown-Chicago headquarters of the daily-deals Web site Groupon. Andrew Mason, the 30-year-old founder and chief executive of what Forbes deemed “the fastest growing company ever,” was standing in the company’s glass-paneled cafeteria on the sixth floor, chatting with employees and popping blueberries into his mouth. Next to him was Spice, a spotted pony with a poofy Groupon-green taffeta bow wrapped around his neck. Spice was a gift for Mayor Bloomberg.

Mason, whose six-foot frame belies his pixie-ish personality, was wearing his usual jeans and long-sleeved button-down. His mop of brown hair was messy, his face stubbly. He looked like he was going to a baseball game, not meeting the mayor of New York.

“Spank the pony!” yelled one employee. Laughter erupted.

“No gawking, please! I thought everyone got an e-mail saying not to gawk at the pony,” Mason boomed. “Can we get everyone out of here? All humans out. Don’t worry about the shit.”

The onlookers dutifully scattered.

An employee walked up to Mason and politely sought guidance on how to present the pony to the mayor. “I really haven’t figured out the narrative yet,” he said.

He had originally planned to give the mayor a puppy, but decided that a pony would be even more memorable. “I mean, it’s such a heavy thing to gift someone,” he said, laughing. “I thought it would be funny to give it to somebody as busy as the mayor.”

He shrugged his shoulders. “Really, I don’t know how it ended up this way. Sometimes I put an idea out there. It’s like telephone, the game—it comes out the other side a pony in the cafeteria.”

Less than 20 minutes later I walked by the cafeteria and Spice was gone.

Moments before Bloomberg arrived, one of Mason’s employees had Googled “horse” and “Mayor Bloomberg.” He discovered that the mayor’s daughter had recently been in a riding accident.

Mason panicked. Worried that Spice would offend the mayor, he ordered somebody to hide the pony. Spice spent the duration of the mayor’s visit in a freight elevator.

“Judgment,” Mason said, winking and pointing to his head.

Two years ago, Mason wouldn’t have thought twice about giving the pony to Bloomberg—awkward moments be damned. He’s the type of guy who comes up with wild ideas that often have no beginning or end, and then lets them rip. Finding out where—or if—they land is part of the fun.

Sometimes Mason gets lucky and finds that he masterminded a killer joke. Other times he ends up with his foot in his mouth. Earlier this year, he offended viewers around the globe when Groupon ran a series of Super Bowl television ads poking fun at deforestation, whales, and the Free Tibet movement. Mason originally defended the commercials, saying they were intended to raise awareness. But, after a barrage of criticism, he apologized and ended Groupon’s relationship with the agency that had executed the ads. Mason now admits that he “flew too close to the sun.”

“If you don’t have those moments where you go too far, then you’re probably not going far enough,” he told a roomful of employees recently.

The deal-of-the-day start-up that Mason launched less than three years ago has certainly gone far. It is now a multi-national corporation with 83 million subscribers worldwide and more than 7,000 employees.

In early June, Groupon made a splash when it filed for its highly anticipated initial public offering, which aims to raise at least $750 million. Once shares start trading, the company could be worth around $20 billion, according to projections. Mason, who controls 7.7 percent of the company, will almost certainly become a billionaire overnight.