As with so many trends, it seemed to happen overnight. One day, no one had ever heard of a puggle. The next, little dogs with furrowed foreheads and floppy ears were everywhere.

For Wallace Havens, however, the popularity of the puggle was a long time coming. In 1975, Havens was at his kennel in Madison, Wisconsin, breeding cockapoos—a curly-haired mix of cocker spaniel and miniature poodle that originated in the 1960s—when one of his customers mentioned another dog he had seen. “He started telling me about his neighbor who had a beagle that got bred to a pug,” Havens says. “Said it was the cutest dog he’d ever seen and if I ever bred them to give him a call.”

Seventeen years later, Havens had a female beagle and a male pug at his kennel and decided to try breeding them. The result was his first litter of puggles, black and tan dogs with bodies like beagles and the trademark bulging eyes of the pug. He was the first person to register the breed with the American Canine Hybrid Club—as the American Kennel Club registers purebred litters, the A.C.H.C., founded in 1992, keeps track of the parentage of hybrids—and so got naming rights. He now sells puggles for anywhere from $450 to $600; pet stores price them at $1,000 to $1,300.

Puggles were not a huge seller until 2005, when they were given the ultimate endorsement—celebrity owners. “Sylvester Stallone got one and he contacted us and had us save another one for him that was lighter in color,” Havens says. “Then, that guy Tony Soprano [James Gandolfini] got one.” In November of that year, the dogs appeared on the Today show, Good Morning America, and the cover of the New York Post. Gary Garner, president of the A.C.H.C., says the number of registered puggles, which began to rise in 2001, increased dramatically. “We keep waiting for the number to taper off,” he says. “But it just hasn’t happened yet.” Now, about 20 percent of the 500 to 700 litters that the A.C.H.C. registers each month are puggles, making it the most popular hybrid. Havens alone has bred 31 litters in the past eight months, making him the largest puggle breeder registered with the A.C.H.C.

The rise of hybrids has been just as dramatic. In 2000, the A.C.H.C. registered about 200 litters a month; in 2007 it is registering 500 to 700 a month. The number of different types of hybrids has also continued to grow—there are now over 400 hybrids registered with the A.C.H.C., including affenpoos (affenpinscher plus poodle), pookimos (American Eskimo dog plus poodle ), rat-a-paps (American rat terrier plus papillon), bordoodles (border collie plus poodle), and Jack-a-bees (beagle plus Jack Russell terrier).

Garner attributes the popularity of designer dogs to people’s desire for novelty—celebrity owners also play a large part. But he also claims that there are real advantages to owning a hybrid. For one, you can tailor a dog to fit your needs. Hybrids are not simple mutts, as some critics charge, but the puppies of pedigreed dogs, bred to combine specific appealing traits of the purebreds. Want a Scottish terrier but have allergies? A schipperpoo might work, since a poodle mix will likely produce a dog that is hypoallergenic. Love rottweilers but don’t have the room? Havens breeds a min-rott that won’t grow to be more than 15 pounds.