It's a sunny afternoon South of Market when Lara Jo Regan arrives at the Eagle Tavern, plops her black carrying case on the patio bar and unzips the top. A small, puffy head peeks out. A bar patron with a buffed chest and pierced nipples glances up from his pint and does a double take.

"Is that Mr. Winkle?" he asks, and when the bundle turns to regard him through unmistakably huge, unblinking eyes, his face brightens with delighted wonder.

Yes, it is Mr. Winkle, touted as the Cutest Dog in the Universe, making an afternoon pit stop en route to an appearance at a Borders bookstore. In a flash, the diminutive ball of fluff is surrounded by cooing men in leather chaps, all of them asking the question that has come to define Mr. Winkle's celebrity:

What is Mr. Winkle, really? An unusually lively stuffed toy? A divine manifestation? The fifth Beatle? A cat in a dog's suit?

Everyone has a different theory about the tiny pooch with an oversize tongue, whose image in calendars and books has launched an international cult of cuteness. More than 36 million people have visited his Web site (www.mrwinkle.com) since its launch in 2000. He averages 100 e-mails a day, and his bookstore appearances are standing-room only.

All this, thanks to photographer Regan's talent for capturing her uber- dog's whimsical charm in guises ranging from angel and alien to ballerina and bumblebee. For her latest book of portraits, "A Winkle in Time: Mr. Winkle Celebrates the Underdogs of History," she poses Mr. Winkle as unsung innovators such as sculptor Camille Claudel and Mexican revolutionary Miguel Hidalgo. She figures celebrating the achievements of history's forgotten heroes is perfect for the world's most celebrated misfit mutt.

"Mr. Winkle is an underdog," she explains. "He was found half-dead on the side of the road and had clearly been abused in the life he came from, but he went from straydom to stardom."

Regan's Winkle odyssey began six years ago when she lost her way in an industrial wasteland outside Bakersfield while on assignment for Newsweek magazine. A bundle of muddy, matted fur limped into her car's headlights; she got out to investigate, and the bedraggled creature toddled into her arms. Feeling the tug of destiny, she took him back to her Los Angeles home and spent the next two years nursing him to health.

MIXED OPINIONS

Though her boyfriend opined that the future Mr. Winkle looked like a demon, Regan sensed that her small charge was more an angel with a dirty face. "I was fascinated by his chameleon-like quality," she recalls. "He looks like so many different creatures: He's got the body of an ostrich egg and the legs of a newborn doe; he has a koala's ears and these huge, otherworldly eyes."

No one knows exactly what produced the 5 pounds of fur and tongue (the mouth muscle is always on display because it doesn't fit into his mouth) that is Mr. Winkle, but many have guessed. A repairman declared him "a robotic squirrel"; a Japanese tourist decided he was "a furry smile." One visitor to the Web site wrote that he must be "an angel of the highest order."

Those and other creative hypotheses inspired the first "What Is Mr. Winkle?" photos and calendar. Five calendars and three books later, fans are still debating his identity: A very small secret agent? A rare breed of dust bunny? (For the record, Regan thinks he's just a unique amalgam of several toy dog breeds.)

Wednesday evening at the Union Square Borders store, 100 fans -- parents and children, Goth girls, skater boys, suited professionals -- ignore Harry Potter displays to marvel at the magic that is Winkle. San Franciscan Ted Guggenheim, 34, waits in line for over an hour to have his calendar signed; Alameda denizens Linda and Rick Romine, 41 and 47, proudly display their custom-made WINKLE license plate.

WINKLE WORSHIP

Debbie Daly, 46, has driven from San Jose to pay homage to the wonder dog. "I would have come from greater distances to see Mr. Winkle," she says. "You can tell by looking at him that he has a special soul."

Regan concurs. "I feel like I'm the keeper of a magical elf. People see him in stores and start screaming. I'll take him to a party, and what was a staid affair is suddenly full of happiness and whimsy. It's almost as if he knows it's his mission to enchant people."

The enchantment has brought much fame and some fortune. But few seeing the symbiotic relationship between photographer and muse believe that Regan, an award-winning photographer for Time, Newsweek and Life before her fateful drive to Bakersfield, wants to exploit the 8-year-old Winkle. This isn't a marriage of convenience; it's love.

"I've fought really fiercely not to sell him out in a crass way, because I feel that he has to be shared in a manner that's tasteful and worthy of him," says Regan, who has photographed more than 100 different Winkle personas. "I want him to be like Winnie the Pooh, a character who's around even after he himself is gone. Right now I've got a big library of images that should last a long time, so I think I'm almost done."

Regan falls silent as she contemplates a world without Winkle. Then she locks eyes with her dog. He gazes back beatifically. She melts. "What can I say? We're obsessed with each other. He's my best bud. This adorable creature with his tongue hanging out, staring at me lovingly. What more does a girl need?"

E-mail Neva Chonin at nchonin@sfchronicle.com.