The SSPCA was alerted after a member of the public called to report four horses with serious injuries on Sunday, September 5.
Three of the horses have serious injuries but are expected to make a full recovery.
Sadly, one remains in a critical condition after sustaining a severe wound to one of their legs.
Inspector Heather Lawson said: “We’re appealing to members of the public to come forward if they have any information about this incident.
“Four of the horses in the field sustained serious injuries and other horses were distressed and agitated, suggesting they had also been chased.
“On examination by a vet the horses were found to have bite and puncture wounds consistent with an attack by an animal, most likely a dog or multiple dogs.
“The incident is thought to have occurred roughly between 6pm on Saturday, September 4, and 8am on Sunday, September 5.
“We know people were camping in the area and using the facilities so we hope someone has witnessed or heard something.
“Being attacked or chased by a dog will have led to these horses suffering considerable amounts of pain and fear.
“We would remind owners that it is an offence for your dog to be out of control in any public or private place.”
If you have information, call the SSPCA’s animal helpline on 03000 999 999 in the strictest confidence.