The point of training a horse to load is so that they want to. At least that’s my point of view. This means it’s not just training. It’s creating a succession of experiences that allows the horse to adjust to and enjoy loading in their own time and head. You can lead the horse onto the box and just go. And a calm and confident personality may well get used to it and never have any objection. If they are cool and logical enough to discover there is fun at the end of the journey then so much the better. But not every horse is a happy customer, they might be naturally timid and such experiences would make them sensitive to travelling and even phobic. Others may have a traumatic experience and experience fear every time they are presented with the box.
In these instances, systematic desensitisation and counter conditioning can be your friends, especially when you allow the horse to dictate the pace of their new learning and make it possible for them to explore. The first is to simply make it a lot easier to be near the box – just graze or eat a meal near it without making the horse feel afraid, or so if they are spooked they can return to eating. Over time the meal is enjoyed over progressive steps onto the rampand into the lorry. The counter conditioning part is the meal itself, the horse chews and enjoys emotions incompatible with fear, so the two events of desensitisation and counter conditioning run together, slowly.
The images and film here demonstrate what that might look like.
Please note the vehicle is secured on blocks!