The assaults, which usually occur in grazing pastures, have reached a peak in the last three months, but span 15 years and according to local vets have involved hundreds of horses.
Some have been stabbed; others have suffered sexual injuries from knives, hammers and blunt objects. Because many of the attacks took place on or near a full moon, Hampshire police speculated that a cult or satanic worshippers could be responsible.
Eleven such assaults have been reported since March, mostly in the Meon Valley, a tranquil area south-east of Winchester, where some residents have started 'Horsewatch' schemes. Police are liaising with vets and riding clubs, but admit they have no idea who is carrying out the attacks - or why. One of the most recent attacks was on two horses owned by Davy Jones, the actor and former singer with The Monkees, who has a house near Droxford in the valley.
Annie, a thoroughbred mare, was stabbed in the shoulder on 13 July while grazing in a paddock at Shirrell Heath, three miles from the Jones's house. The following night - when there was a full moon - Chrissy, a palomino mare, was attacked in a field next to the house. A knife or similar sharp instrument had been used to cut her hind quarters, back legs and genitals. A nine-inch cut had been made to her genitals, and an object, perhaps a broom handle or a fence post, had been forced inside her vagina.
Anita Jones discovered the assault and called a vet. 'I was horrified. It was not just the injuries. It was the fact that somebody could actually do something as cruel as that to a horse,' she said. Both horses have recovered, but Chrissy, a pet for their two daughters, is still traumatised and badly swollen.
One of the first attacks this year was in April, 10 miles away at Alton. Millie, a 15-year-old thoroughbred mare belonging to Olive and David Gray, was savagely assaulted overnight. The attacker avoided security devices to enter a locked stable 400 yards (365m) from the house. He or she selected Millie, the only mare in the stable, cut her rear quarters and left evidence of an attempt to force a blunt object into her vagina. Millie was treated by a vet but became ill and was put down a month later.
Mrs Gray said: 'She was never the same horse after the attack. She just deteriorated. You cannot imagine the mind of someone who can do a thing like that. It was like losing a baby.'
Vets in Hampshire are familiar with the problem. Andrew Kennedy, who treated the Jones's horses, said he first became aware of the attacks 15 years ago, and had seen more than 30 injured animals since. His colleagues tell of similar numbers, suggesting that hundreds have been attacked.
Clusters of attacks have been reported in different areas of the county, usually in the summer months, with the last 'peak' in 1983/4.
Theories about the attacks abound. Most horse owners agree that more than one person would have to be involved, and they would have some knowledge of equine physiology.
Police have investigated theories that the attacker may have a grudge aginst horses and may work with them, perhaps as a vet, stable boy, farrier or trainer. Some time ago, an elderly man was questioned over the offences and was placed in a psychiatric hospital. The police believed they had got their man, until the attacks resumed.
Police have no idea if one person is responsible, or if there have been 'copycat' crimes. The only common factors are that the attacks take place in fields or stables near roads, footpaths or bridleways.
The only leads this year involve local newspapers - horses that had been attacked had been advertised for sale in one, and soon after an attack on two horses in Romsey, a man telephoned another local newspaper with significant, but misleading, information. Police had not released any details of the attack and believe the man may have been involved.
A police spokesman said: 'Feelings are running very high. There is no knowing what will happen if they catch this man before the police do.'