I went to an emergency rescue lecture and they told me two must-haves, they are mostly for accidents.
#1 a tow strap at least 4" wide, 6" would be better, and at least 25' long, 30 is better. This is for dragging a horse out of a trailer if necessary, they also showed us how to do it. I also have one in the barn for the same reason. NEVER PULL THE HORSE BY THE LEGS, HEAD, OR TAIL.
#2 A roll of yellow caution tape. In case you have an accident, the first thing is to contain the area, keeps people out. Also if the horse gets loose, most horses now will relate the tape to electric fence, so it will contain the horse also.
The lecturer told several stories of the owners getting the terrified horse out of the trailer, only to have it get loose and run miles down the hwy.
I also never travel without a camping jug, 7 gallons, of water. I keep it filled and in the trailer, and empty it and refill before traveling. That way if I need to go in an emergency, I have water, stale water, but water.
One more thing I learned from the dealer that sold me my first trailer, Never let anyone else hook or unhook your trailer. I did a terrible thing once that could have been a horrific accident, but since I was the only one that hooks up my trailer, I would have only myself to blame. When I found out what I did, I literally got on my knees and said some prayers and thanked God that he was watching over me me that day.