"The problem of fly-grazing - the problem of horses on common land - is an enormous problem" says the RSPCA's Gethin Russell-Jones.
"Our RSPCA inspectors are involved daily in recovering horses, dealing with complaints".
"There must be hundreds and hundreds of horses scattered across South Wales at the moment".
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Fire crews have been responding to reports of flash flooding in Carmarthenshire. Some homes have been affected.
Medication designed for animals was taken from the building in Abertillery on Thursday night.
More than a thousand riders will be taking part in a series of events in and around the town.