Law enforcement officials said a low-caliber rifle may have been used to carry out the massacre.
Thirty or more of the horses were known to graze the quiet fields, according to residents who live near the site.
Witnesses said one of the dead animals was a yearling that had been shot in the face.
At least two of the other horses were pregnant but miscarried after being slain, according to reports.
Hunt said one of the dead still had grass in its mouth.
Local veterinarians were performing necropsies to determine the official cause of the deaths.
“Seeing them gunned down is … it’s beyond horrific,” said Tonya Conn of local group Dumas Rescue, which has helped care for the horses in the past and is now helping the police search the woods for the dead animals.
"The response across the country to this horrific situation has restored our faith in humanity," the group said in a statement Thursday on Facebook.
Since the first reports of the massacre, donations have poured in from across the country, raising the initial reward from $1,500 to $15,000 in a matter of days.
The Humane Society of the United States also put up a $5,000 reward.
On Thursday, investigators went back to the scene to collect more evidence.
Hunt described the shootings as one of worst cases of animal cruelty he has ever seen.
"This is very inhumane," he said, "and it's a very cruel act of somebody who just apparently had nothing else to do or whatever, just to go back on a strip job and shoot down horses..."
In November, President Donald Trump signed a bill into law making certain types of animal cruelty a federal felony.
»RELATED: Trumps signs bill to crack down on animal cruelty
The Preventing Animal Cruelty and Torture Act makes intentional acts of cruelty to animals federal crimes, carrying penalties of up to seven years in prison. The legislation expanded a 2010 law that made creation or distribution of “animal crushing” videos illegal. The Senate unanimously passed the Preventing Animal Cruelty and Torture Act on Nov. 6, two weeks after the House passed it on a voice vote.