The 1917 Fowler engine, worth £250,000, came off the trailer in Crays Hill, Essex just missing an electric pylon.
The engine was being returned from its inaugural appearance, following a full restoration, at Young Farmers event near Chelmsford.
"The primary thing is that no-one was hurt," said the owners.
Phil Burrows, from Barleylands farm park, who owns the Panther said they were "unsure" what caused the incident at about 19:00 BST on Sunday.
"I got the scene and wanted to make sure no-one was hurt and the lorry diver was OK. Machines and metal can be replaced but people can't.
"The driver was devastated and the chap who looks after it was a bit distraught at the state of his poor old engine."
The engine had been "lovingly restored from a wreck to working condition" over the last five years, had "just finished its repair" and was on "its first outing before our country show in September," said Mr Burrows.
"It took about two hours with three cranes to right it and then another hour to get it onto the low-loader.
"All the steering arms were bent so you couldn't steer it. Due to its age and value, we had to spend a lot of time working out how to lift it as not to exacerbate the damage.
"With a steam engine you cant just buy something off the shelf everything has to be handmade so a lot of craft work and skill goes into it."
The engine's flywheel, boiler and steering mechanism were damaged in the accident.
A 21-tonne steam engine that tipped off a low-loader and "slid down a road" could cost more than £100,000 to repair, its owners said.