The cob was one of three horses to let themselves out through a footpath gate left open by someone walking through the land, near to the A26.
Cliffe Equine Clinic, in Lewes, is now urging people to be more attentive when crossing an area used by livestock.
A message posted on the clinic’s Facebook page said: “I personally would like to issue a word of caution to anyone who uses footpaths through land containing livestock - if you open a gate, CLOSE it behind you.
“Today, we sadly lost a beautiful cob on the A26 in a RTA, one of three horses that let themselves out through a footpath gate left open by the last user.
“The car that hit the horse was a complete write-off, but very luckily the driver was uninjured.
“This was a wholly avoidable tragedy and the owners are devastated.
“Please share to stop this type of tragedy, or worse, happening to anyone else.”
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