Two horses were killed after being hit by an articulated lorry when they escaped from a field onto a motorway.
The accident happened on the anti-clockwise carriageway of the M25 at Swanley in Kent early on Wednesday.
Emergency services were called to the scene after witnesses reported seeing four horses loose on the motorway but when they arrived two had been hit.
The lorry driver was uninjured but the M25 was closed until mid-afternoon.
Traffic police said it was lucky a more serious accident did not occur.
Insp Geoff Wood, of Kent Police, said the horses walked onto the motorway via the slip road just after 0500 BST.
He said two other motorists suffered slight injuries when their vehicles ploughed into the debris caused by the initial accident.
Junction three of the motorway was closed for more than nine hours, with traffic tailing back to before junction four.
Vehicles were diverted off the junction for Swanley and back on the M25 again via the on and off slip roads.