The owners of Dale were banned from keeping horses for five years and fined £500 each after they allowed the Shetland pony to become grossly overweight.
The charity today released findings of a major survey indicating the public has trouble recognising an obese horse and the health concerns that can generate.
More than half of 2150 members of the general public surveyed either did not correctly recognise an overweight horse or, if they did, indicated that it posed little or no welfare threat.
"We are a nation of animal lovers, but sometimes we can love our pets too much and with devastating consequences," said the charity's chief executive, Roly Owers.
Around half of all companion animals are now obese in Western civilization, the charity said.
Other research indicates that 35 per cent to 45 per cent of British horses are believed to be overweight or obese.
The chairty said its own research, combined with the feedback from its 16 field officers, has provided what it calls a unique insight into what the public perceive to be the main problems and what is happening in reality.
"This research is vitally important for horse welfare," said Owers, "but the results are worrying.
Overfeeding a horse can lead to ongoing health issues such as painful laminitis, and heart and lung problems. Horses can also suffer from a diabetes-like condition called Equine Metabolic Syndrome, which can be as equally destructive as it is with humans.
"When asked to investigate concerns involving thin horses, our field officers often arrive to find that the horse in question is actually the correct weight, but looks thin compared its overweight field companions."
The charity cited the penalties imposed on a married couple from Blackpool last month, who were banned from keeping horses for five years and fined £500 each after they allowed a Shetland pony in their care to become grossly overweight.
The pony, named Dale, was rehabilitated by the charity but will be at risk from weight gain and laminitis for the rest of its life.
"This prosecution is likely to be the tip of the iceberg, as many similar cases could reach the courts in the future," said Owers.
"We hope that cases such as this will serve to highlight an escalating problem in our horse population, which if left unchecked will have dire consequences."
He said the charity had been weighing and assessing horses up and down the country and educating their owners over the last two years through its Right Weight project.
"We will continue to lead the way in finding solutions to combat the key causes of horse welfare problems."
Among the survey respondents, 86 per cent recognised an underweight horse and 73 per cent recognised an overweight horse. In all, 76 per cent believed the underweight horse's welfare to be at risk.
However, only 46 per cent believed the overweight horse's welfare to be at risk.
Other findings included: