Dec. 20--NEW HAVEN -- A mounted police officer was thrown from her horse near Edgewood Park Tuesday after an agitated dog broke free from its leash and attacked.Officer Kim Roche was shaken up, but not seriously injured after the experience. Her horse, Marshmallow, suffered puncture wounds to the nose, legs and genital area and was going to be checked out by a veterinarian.According to police, the attack happened Tuesday afternoon while Roche was on patrol in Edgewood Park and continued for blocks as the riderless horse ran.A horse breeder said adog-on-horse attack is almost as unusual as it must have been terrifying."All horse owners have dogs of all different breeds. Sometimes they get ticked off at a horse, but (never) actually go after a horse and attack it," said Martha Wetmore, who raises, trains and breeds horses at her Bittersweet Farm in Bethany. "I don't think I have ever seen a dog go after a horse."She's been in the horse business since 1953.At times, while Wetmore is driving horses through residential areas, family pets come charging out but always stop short, apparently surmising discretion is the better part of valor, she said.The encounter Tuesday started pleasantly enough. Roche was on patrol and stopped to chat with a woman walking two boxers, according to police Sgt. Russell See Jr. They spoke for a time. Roche started to ride off after noticing the dogs getting skittish, and they got more and more excited as she left."The woman was trying to hold the dogs back, and one of them broke the leash and went after the horse. The officer was thrown," said See.The horse trotted off, with the dog tenaciously pursuing and biting it, and both took a varied route that ended up with police corralling them a block apart near the intersection of Elm Street and Sherman Avenue, some 10 blocks away, police said.Along the way, motorists swerved to avoid the Wild West scene that played out pretty much down the middle of the streets, police said.Police checked that the dog's license and vaccinations were up to date and released the animal to its owner, who was not charged.Roche went back to the stable with Marshmallow and was examined by a doctor later, See said.An experienced equestrian, Roche is the Police Department's former stable manager and became a police officer in 1997. She had been partnered with Marshmallow, a large Percheron-cross, since this summer.Constantine "Gus" Marnel, a dog trainer and owner of K-9 Communications in Milford, said it's impossible to say what triggered the attack."There could have been a million trigger responses, but bottom line is there was an inability on the part of the owner to control her dog," he said.Marnel said it sounded like a "fear-biting scenario," although he couldn't be certain because he wasn't there.It's not uncommon for dogs to wait until a perceived threat is leaving to bite, he said."With two dogs, they may have been thinking they're protecting their owner or each other and were waiting for the opportune time to bite the horse," he said.