The RSPCA, which was contacted by people who were concerned about the animals in Bridgend, said an investigator would assess the situation.
"They have apparently been wandering around for the last few days," Bridgend resident Pat Jones told the BBC
"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."
reported that five of the horses were struck by cars and killed and others are starving.
page called People Against the Neglect and Abandonment of Horses was set up to spread information about what is taking place.
"Currently, there are approximately 250 horses living in terrible conditions in Bridgend, having been left to fend for themselves," states the information on the page. "Though they apparently have an owner, it's hard to describe this as anything other than both abandonment AND neglect. Currently, 5 are known to have died, at least one of which still rots where it fell."
The group believes the problem is widespread, but is especially concerned about the animals in Bridgend because of the lack of attention given to the problem, and a lack of action on the part of authorities.
They said that, along with concern for the horses, there are concerns for road safety and environmental health (because of horses left to decay).
"The Monmouth based Society for the Welfare of Horses and Ponies have been working hard to try to come to a solution, and have even called out the RSPCA, who shockingly, have done little if anything about this situation," states the information on Facebook.
"The SWHP have also been in negotiations with the owner to try to rescue at least a few of these animals, but the scale of this issue dwarfs the organisation, and progress, if any, is very slow."
of the horses have also been posted online.
A road in South Wales was closed for most of the first weekend in January
because of about 70 horses running loose around Llanelli. They caused damage to parks, football pitches, and a golf course.