It happened at about 9am yesterday in Toulston Lane in Bramham.
The rider, a woman in her sixties, was seriously injured after being hit by a silver Vauxhall Astra.
She was taken to Leeds General Infirmary by air ambulance.
She remains in hospital.
The horse was also seriously injured in the crash.
It was put down by a vet at the scene.
Another woman, who had been riding with her, received slight injuries after she was thrown from her horse.
The driver of the Astra has volinarily spoken with police.
Officers are asking anyone who saw the crash or who has relevant dashcam footage to contact PC 6100 Beecham at West Yorkshire Police Roads Policing Unit via 101 quoting reference 13190640544.