The story about trailer safety and disasters was great-but I would like to add one more thing-TIRES! 

My daughter and I drove 2 hours to pick up a horse that a lady could no longer keep.  She hadn't been ridden for over a year and was under nourished some.  The trailer was an older used trailer in mint condition.  The tires looked new as they weren't worn at all.  Wehad just purchased this trailer a few months earlier.   On the way back (the day was warm) all of a sudden one of the tires on the driver side of the trailer blew.  The tire just blew apart.  After an 1 1/2 hours we got a tow service to jack up the trailer and put the spare on for us; without taking the mare out of the trailer.  She was perfect altho she hadn't been out in a couple of years.  We didn't want to unload her along the busy highway with out knowing how she would act.  Now, back on the road happy as could be that it only cost us $97.00 cash to get the tire change.  We didn't get 5 miles further when we "smelled rubber burning".  We pulled over and when I check the tires I found on the right side one of the tires was beginning to separate.  Sooo, we drove to a gas station (slowly)about a mile further.  I had to call my husband and have him bring my horse trailer so we could put another spare tire on the trailer, transfer the mare to my trailer and limp home.  A 5 hour trip turned into 8 hours, took 2 spare tires, AND could have been a disaster.  Warning:  whether or not the tires look like they have good tread, if they are more than 5 years old they need to be replaced.  We learned a good lesson.  Thankfully nothing happened to the mare or us.