The 56 year-old was walking with a woman along the grass verge on the A171 Guisborough Road when the collision occurred yesterday (Wednesday) near to the Newholm junction at around 3.25pm.
He suffered arm and head injuries and was taken by ambulance to James Cook Hospital in Middlesbrough where North Yorkshire Police say he is in a "serious but stable" condition.
Police are appealing to witnesses who saw the manner of driving of the horse box prior to or at the time of the collision, as well as the man and woman who were walking near the road, to get in touch.
Officers are also appealing to anyone who may have dashcam footage of the collision.
Contact North Yorkshire Police on 101, select option 2, and ask for Paul Moon. Or email [email protected]