Five horses died after running loose on roads including the A1(M) in County Durham.
Police said 19 horses escaped from a field next to stables in Nunstainton near Bishop Auckland early on Tuesday when a gate was forced and left open.
Three died after being hit by vehicles and one was injured and later put down.
Hours later a horse owned by a travelling family was hit by a car in Tindale Crescent, Bishop Auckland, and was put down.
Crashes were reported on the motorway between junctions 60 and 61, and on the A688 and A689, Durham Police said.
The affected stretch of the A1(M) was closed for about four hours following "multiple calls" from motorists.
Police said the remaining horses were recovered, but all had sustained injuries.