The accident happened on the Lisnevenagh Road, Ballymena,
It’s understood a Vauxhall Vectra was involved in a collision with a horse on the carriageway shortly after 4am.
A second vehicle, a Volkswagen Bora, is then believed to have struck the animal.
Inspector Peter Duncan said: “The driver of the Vauxhall has sustained injuries that while serious, are not thought to be life threatening.
“The five occupants of the second car sustained only minor injuries.
“The horse was killed in the crash.
“Police investigating the collision are appealing for witnesses. Officers can be contacted on 101, quoting reference number 225 13/01/18.”
The road remains closed at present.