The crash happened at Sproughton, near Ipswich, at 22:20 GMT on Christmas Eve after the horses had got loose.
A 26-year-old Ipswich man was later arrested on suspicion of causing danger to road users.
Paramedics took the injured man to Ipswich Hospital. Police said he had life-threatening injuries.
A section of the westbound A14 between Copdock and Sproughton was closed for several hours in order to recover the vehicles and horses.
The arrested man was questioned at Martlesham police station.
