The first suggestion I would make is that you use a strap halter rather a rope halter when tying your horse. The rope halter can tear into the poll area and injure the skin or worse when he pulls back.
The previous owner gave you what may be some valuable info--your horse may be still be scared of the trailer tires and when anything noisy, etc., happens it might make the tires move just enough so that he reacts as he did when the cat frightened him. What you might try is to put something like a piece of plywood the same color as the trailer between the fender and the tires and make sure it won't move around by blocking it well or screwing it tightly to the fender.
I just read Temple Grandin's latest book "Animails in Translation" about her autism and animal behavior in which she describes cattle (her speciality) being frightened by things that aren't always apparent to their handlers. To complicate the situation by now your horse may have become fearful of any sudden movement or noise such as the manger door closing suddenly, etc.
Well applied sacking out in a safe area may also help--like putting a piece of plastic on the end of a whip and gradually getting your horse used to it being moved around and tapped on something which also will get him used to a noise. I hope you are able to solve this problem--it is a tough one.
