I have a 4.5 yr old Arabian gelding that has been trailered many times. Recently, we started taking him to a training facility where he is being started under saddle. The first week of training he loaded fine. At week two, he became slightly hesitant. Before we start week three, we thought it would be good for him if we took a couple of days to just load in the trailer and go to a fun place where he could graze and run. But when we tried to load him today, he refused to get in the trailer.
We tried treats, and other means such as rest/work, but nothing worked. Tomorrow we will take the whole day to work on this because he has a full week of training sessions.
We are very careful slow drivers so he's had no bad experience there. I'm fairly confident that he's associating the trailer with his training.