A horse charity has said a number of the animals dumped in Nottinghamshire probably came from the same stable.
Four more horses have been abandoned in the past week, after four were deserted in February, including one which died after it was thrown out of a trailer.
Janice Dixon, a vet at Help for Horses, said they all appear to have fallen ill from the same stomach disease.
The RSPCA is investigating, but said it was struggling to cope with an "equine crisis" across England and Wales.
The animal welfare charity said it gets about 80 calls a day on its cruelty line from people concerned about horses.
Three piebald mares, in a "poor bodily condition", were left in a field near Park Lane, in Kirkby-in-Ashfield, on Sunday.
One of them had to be euthanised, while the other two have been receiving veterinary care.
It followed a horse being left in Pinxton Lane, Kirkby-in-Ashfield, on 7 March.
The landowner said she was bullied by a gang which left the animal on her field.
Police said they were called to the site after a report of criminal damage.
Ms Dixon said the dumping of horses was despicable animal cruelty and a crisis of "epidemic proportions".
She said the cases in Nottinghamshire were related and dying horses were being abandoned because it is cheaper than getting help for them.
The vet said she expected more horses suffering from malnutrition and diarrhoea to be abandoned.
The RSPCA and Nottinghamshire Police have appealed for more information.