RSPCA inspectors are investigating reports that up to 250 stray horses are running wild in Bridgend.
The alarm was raised after a member of the public said many of them appeared to be emaciated and starving.
Some of the horses are said to have been found dead while others continue to wander between Coity and Laleston.
An RSPCA spokesperson said an inspector will be sent out to assess the situation.
The charity was contacted by people concerned that the horses have been abandoned and are in a poor physical condition.
Hundreds of people have been posting on a Facebook page set up by the Society for the Welfare of Horses and Ponies, expressing their concern at the horses' condition.
Chair Jenny Macgregor said it was an ongoing problem: "The horses are everywhere, we've had reports of them getting onto the M4 at Pencoed and being killed.
'Out of control'
"We have found at least five or six dead and they are usually yearlings dying of starvation and lung infections.
"The situation is out of control."
Pat Jones from Bridgend told BBC Wales: "They have apparently been wandering around for the last few days.
"One of my friends has spent the morning trying to keep them from running into the road at Penyfai.
"There are around 250, but they have split up into groups and they are absolutely starving.
"On Facebook people are saying at least five or six have been found dead. It's really sad."