A horse sanctuary in Essex said it had been "inundated" with thin and starving animals during the recession.
The founder of the Remus Memorial Horse Sanctuary in Buttsbury, Sue Burton, said the rise was due to people not being able to afford to keep them.
Ms Burton said: "We have not seen cruelty and suffering like this for over 25 years and I believe things will get worse before they improve."
Two emaciated ponies brought in to the sanctuary in the past month have died.
'Left to die'
Ms Burton said it seemed to be a UK-wide problem.
She said one of the mares they found was so thin her spine could be seen.
Some horses were found by the sanctuary in Colchester left out in snow without any food or shelter.
She said: "We have been inundated with people asking us to take their horses because they can not afford them or can not afford their vets' bills.
"We have taken in several with horrendous health problems that have just been dumped in fields and ostensibly left to die."
She urged anyone with concerns about an animal to report it and to particularly watch out for tethered animals, as they may not be able to reach food or shelter.