Good question. They were not yearlings, but I just bought two young mares that had been loaded exactly once each. One of them had been to the vet for a pre-purchase exam and the other had never been in a moving trailer.
I bought two because I thought they'd do better with the transition that way. The good news is that even though they were in separate pastures, we had them loaded and in the trailer and were back on the road in 17 mintes flat. You've just gotta love Icelandics. On the other side of that picture, I have an intense little Tennessee Walker gelding who is not a great loader. He does better if there is a horse in the trailer already and hauls better if he has company. We're endurance riders and often have a 5 or 6 hour drive. One time I loaded my grandson's gelding first and he looked around and made a face at Hunter, and it was all over. He was not going in the trailer with the evil Arab. We put a different Arab in the second spot and Hunter loaded up just fine in slot #3. He's a bit of a stinker, but I have no idea of his past. I've had him a couple of years and he has improved at every level over time and with lots of miles under saddle. Nancy
