Step 1: Making the 3D model
Rather than use someone else's model of a Trojan horse, I decided to design my own from scratch. I knew that I wanted the final model to be quite simple and angular, as this would make it easier to deconstruct into a sensible number of cardboard panels. Fortunately, it's much easier to model a stylized low-resolution horse than a realistic and detailed one.
My initial plan was to have enough room in each pair of legs for someone to stand, with the person in the rear bent forward and the person in the front standing upright, much like with a traditional pantomime horse, only much larger. In fact the horse ended up so large that the person at the back would be able to stand upright quite comfortably.
I modeled the horse in Blender, starting with a simple cube and extruding new faces from it until I had a very rough outline of a horse, as shown in the first image here. I then gradually refined the shape of the horse, subdividing the edges where necessary, until I had something much more distinctive. I modeled the ears, the tail and the mane separately, then joined them to the horse with a Boolean operation.
The whole time I was modeling the horse, I was thinking, "Would this look good 50 feet tall and made of wood?" As design briefs go, I quite enjoyed that one.