The incident occurred on the B1263 at the crossroads with Holywell Lane, North Cowton, at about 11.05am on Sunday 31 January 2016.
The 82-year-old man on the bicycle, who is from the Darlington area, entered the main road after approaching the junction from Holywell Lane.
He suffered fatal injuries in the collision with the black and gold horse box which was travelling on the B1263. We are not in a position to name the man at this time.
Police are appealing for people to come forward who witnessed the collision or possibly recall seeing the horsebox or the cyclist prior to the incident.
If you can help, please call North Yorkshire Police on 101, select option 2, and ask for the Major Collision Investigation Team. Or email MCIT@northyorkshire.pnn.police.uk quoting reference number 12160018001.