James Gray, 46, neglected more than 100 animals on his family farm, with horses found crammed into pens ankle-deep in their own faeces and carcasses littering the fields.
Gray attended Aylesbury Crown Court, Buckinghamshire, to appeal against his 24-week prison sentence today, having lost an appeal against his convictions last week.
But when the court resumed after adjourning for the judge to deliberate, Gray was nowhere to be found.
It is thought he walked past security guards and into the town centre, despite being told by Judge Christopher Tyrer not to leave the court building for the hour-long break.
A warrant for his arrest was issued.
Michael Fullerton, representing Gray, was asked by the judge where his client was, and replied: "I have no idea. I haven't seen him from the time the court rose."
Gray, who was convicted along with his wife and three of his children following the neglect at Spindle Farm in Amersham, Buckinghamshire, was sentenced in his absence to 26 weeks in prison.
Sentencing Gray to the maximum possible sentence, Judge Tyrer said: "What the court has been listening to is an horrendous case of animal cruelty.
"It is the worst case ever experienced by the RSPCA. In our judgment, this was animal cruelty on a scale that beggars belief.
"In January 2008, the RSPCA were faced with a calamity of huge proportions. One hundred-plus horses had to be removed. Others were so ill that they had to be euthanised.
"Carcasses in various stages of decomposition were found. Some of them were in full view of the farmhouse, where young children lived and had access to the adjacent garden.
"The animals were dead and dying. There was neglect, starvation, emaciation and squalor. The animals that the RSPCA found were hungry and thirsty, dejected, miserable and depressed.
"After removal, nearly all of them recovered and have thrived.
"The business, of which all of you were party, was concerned only with profit. Animal welfare did not figure at all."
Gray and his teenage son, James Gray Jnr, were found guilty of 11 charges each under the Animal Welfare Act 2006 after a trial last year. Two of the charges were dropped when they launched an appeal following sentencing last June.
James Gray Jnr, 17, was acquitted on two further charges following the appeal, but all other convictions against him, and his relatives, were upheld.
Gray's wife, Julie, 42, and daughters, Jodie, 27, and Cordelia, 21, had each been found guilty of two animal welfare offences.
All of the Grays were banned from keeping horses, ponies and donkeys following the neglect. Gray was banned for life, with his wife, son and daughters each banned for 10 years.
RSPCA officers found hooves and body parts scattered across the site when they visited the farm, along with a mound made up of bones and skulls.
They said the investigation was one of the charity's "biggest ever" and cost more £1.4 million including court costs and veterinary bills for the neglected animals.
In total, 114 horses were removed from the site, many in a state of starvation. Three of those had to be put down. There were also 32 dead horses at the farm.
The animals removed from the farm can now be rehoused, as an order was passed to prevent them being returned to the Grays.
The judge added: "None of you has shown any remorse. You continue to protest your innocence in the face of overwhelming evidence.
"The three female appellants simply tried to distance themselves by pretending a lack of involvement."
Speaking after the sentencing, RSPCA inspector Kirsty Hampton, who investigated the case, said Gray was a coward for failing to return to court to be sentenced.
She said: "I'm really pleased that the sentences have been upheld. James Gray Senior and Junior in particular showed no remorse whatsoever.
"I think it was cowardly to not turn up for his sentence."
She said the abuse was the worst she was ever likely to see in her career.
She added: "It was horrific. They were absolutely filthy conditions. There were dead horses kept with live horses.
"At no point did the family seem to think they had done anything wrong, and that it was anything other than a normal way to keep horses. I was disgusted."
Gray has already been ordered to pay the RSPCA £400,000. A further costs hearing will take place later this year.
The sentences for his other family members were upheld, with Julie Gray and her daughters each given a 150-hour community service order.
Julie Gray and her son and daughters will only be able to reapply to keep horses after seven years, with Gray told he can reapply after 10 years.
Gray, his wife, son and daughter Cordelia are all of Spindle Farm, Amersham, while Jodie Gray lives in Park Road, Ashford, Middlesex.
