The horrific incident, witnessed by children, happened at around 4pm on Sunday in Hill View, next to Ballyguile in Wicklow town.
The horse was one of eight being unloaded into a field when it refused to co-operate.
After repeatedly trying to coax the mare out of the box, a group of men "got extremely violent" with it, said a spokeswoman for the Irish Horse and Welfare Trust.
She said: "When it refused to get out, the car drove off which sent the horse shooting out the back and it collapsed on the road.
"They then beat it to get up and it was beaten quite badly because it was so distressed it wasn't able to get up.
"Gardai were called and the vet came up who determined its injuries were so severe the animal had to be put to sleep.
"People living in the estate witnessed what happened. The situation was quite fraught so we weren't able to speak to the people who we believe owned the horse. It was an extremely barbaric case and the cruelty was unimaginable."
The mare's foal stood over its body until residents covered it with a sheet and called for it to be taken away.
The spokeswoman added: "We're trying to get people who saw it to give a statement so prosecutions can be made. It was a very distressing situation, and gardai are treating this very seriously."
(Independent IE - December 10, 2013)
