Most of my ponies or horses have been reasonable loaders, years ago a strap around the quarters was usually the acceptable method of encouraging them in.
Then I had a youngster off my haflinger mare , the 1st time she was loaded she was 8 months old , into a horse box , to travel with her Mum from the West Midlands up to North of Glasgow. The haflinger was a good loader and the foal just followed Mum in but when we got to our destination after a long slow journey with stops, she refused to come out, even though the haflinger was unloaded. She was eventually half carried out and then took a massive leap, quite scarey for all concerned.
She didn't need to be trailered anywhere after that for some considerable length of time but I had decided to enter her in a showing class so she had to get there somehow.
The owner of the yard was the most patient horsewoman I have known and she helped me each day getting the youngster used to the trailer. Eating a haynet tied to the outside, walking around and sniffing without and pressure to go in. Eventually we wanted to test the water with her so lead her up to the ramp, she had her two front hooves on the ramp and became uncomfortable so she was calmed and walked away, this happened each time so we guessed it was the noise of the ramp combined with a slight unsteadiness.
We used muck and shavings from the muck heap to cover the ramp and made a path way of the muck/shavings leading up, the youngster was allowed to sniff the path and then walked well up the path and right onto the ramp before realising and straight into the trailer.
Now I had expected to tie her up with a hay net and leave her in for a short time but the yard owner advised me to walk her through the trailer and down the side ramp without stopping. I did this and walked the youngster back to the muck path and did the same again and then once more again.
After that she had a reward of a wee bucket of carrot/apple and allowed back into the meadow.
The next day we went through the same procedure and eventually after about 5 days we did tie her up with the hay net but left the side ramp down so she could see out. Eventually over the next few days , we shut the side ramp, then the back went up and eventually she went for a drive around the block.
That pony never ever had an issue with loading /unloading or travelling.
If you are lucky enough to get them young and spend a little time each day for a couple of weeks or so they learn the lesson of not to fear.
I do think the best thing was leading her straight through the trailer and back round , in my opinion that is when she learnt not to fear.