Unweaned foals have been found dumped on council land near Rhoose in the Vale of Glamorgan.
The Society for the Welfare of Horse and Ponies (SWHP) believes five, three-month-old foals were thrown over a fence into the field on Wednesday 3 August.
The SWHP has taken one severely lame filly into care as she has a suspected broken stifle and pelvis.
Lulu II, as she has been named, is currently on a course of antibiotics and painkillers while receiving foal replacer milk every four hours.
Jenny MacGregor, chairman of the society, said: “This really is terrible, these foals should still be with their mothers, taking milk.
“We’ve had a lot of youngsters dumped recently and are really starting to struggle as we have 15 more animals than we normally can look after.
“Sadly, I expect that there will be a lot more dumped over the coming weeks.
“We are just about coping at the moment but with winter on the horizon I really am starting to be concerned about the future.”
The other four foals are currently in the ownership of the council under an enforcement notice.
If they have not been claimed by the end of this then new homes will be found for them.
The SHWP works takes in sick, injured and abused animals and nurses them back to health.