Researchers at Nottingham Trent University, UK, investigated the effect of trailer transportation on horses when traveling alone, with a companion, or with a safety mirror to mimic a companion.

Measures of stress levels were taken during 30 minute trailer sessions with twelve mature horses.  Behaviors which indicated stress levels, such as vocalization, pawing, head-tossing, and looking around, or eating were recorded.  Physical measures such as  temperature and heart rate were also taken.  

It was found that both behavioral and physiological  indications of stress were lower when horses travelled with a companion:  Horses did less vocalizing, head tossing, and pawing, and spent more time eating; they also showed less of an increase in heart rate and rectal temperature, along with less of a decrease in ear-temperature.

Interestingly, when traveling with the mirror placed in the trailer, behavioral indicators were significantly lower than when traveling alone.  Physiological indicators showed slightly lower stress levels than when traveling alone, but not as low as when trailering with a companion.

The authors conclude that travel with a companion if possible is preferable when trailering horses.  Some of the measures, such as ear temperature, had not been applied in equine studies before and should be evaluated further.

For further information, see:  

Rachel Kay and Carol Hall, The Use of a Mirror Reduces Isolation Stress in Horses Being Transported by Trailer.  IN Applied Animal Behaviour, Vol 116, 2 (237-243)