It was "weaving" -- a medical condition brought on by confinement, boredom, and lack of food.
"He had been in there a long, long time," Judge Jane Farish said at Christchurch District Court today.
It was saved after being taken to a safe farm, but six others had to be put down.
At court, the brothers admitted six charges of wilful or ill treatment of animals, five charges of failing to look after them, and one charge of ill treatment of an animal in their care at properties at Halswell, on the southern fringe of Christchurch, and Kirwee further inland.
The SPCA visited the Halswell property at Quaifes Rd in 2009 where they found horses with inadequate feed and water.
Rubbish was scattered around the property that was poorly fencing and overstocked.
The brothers were given three separate statutory notices to give supplementary feed, give them lice and worm treatment, remove hazards around the property, trim their hooves, reduce the number of horses, and supply more water troughs.
But when the SPCA officers returned in March 2010, they found an emaciated horse that could not get up and was crawling on its knees.
It was in severe pain and distress and was shot where it was found.
Of the 34 horses on the 15-acre property, a quarter were found to be in "good" condition, half were in "poor" condition, and the last quarter were in "extremely poor" body condition.
A total of six had to be euthanased.
Post-mortems showed they were starving, three had severe parasitic infestations, one had kidney lesions, and another had lung lesions.
Defence counsel Tim Fournier said both men, who have been around horses their whole lives, have "both expressed disappointment in themselves that they failed the horses".
"People get animals because they like them, not generally because they want to mistreat them."
But Judge Farish said it "went beyond neglect".
Crown prosecutor Ben Vanderkolk raised concern that the Williamsons hadn't shown any sense of responsibility for what happened to the horses.
"The stress and suffering was entirely unnecessary and unreasonable," he said.
Mr Vanderkolk said the offending had put a serious strain on the charitable SPCA's "limited and finite resources", with no hope of getting any money back.
The SPCA, who brought the prosecution, sought reparation of $92,000, after spending $64,000 on veterinary care and $28,000 for feed, board and transport.
Judge Farish said the facts of the case were "disturbing and at times distressing".
She also expressed concern that neither man showed remorse or an understanding of what happened.
They were both jailed for 16 months and ordered to pay $3500 each as a nominal sum to the SPCA.
