Source: Kentucky Equine Research

A survey of German horse owners found that even short-distance transportation of less than two hours can cause significant stress and health problems for horses. Of the 258 respondents who completed the survey, one-third reported behavioral issues such as fear, aggression, reluctance to load, rearing, and flight responses, with problems occurring most often during loading and transit. Horses transported alone were significantly more likely to exhibit these behaviors than those traveling with another horse. Health concerns were reported in 42% of horses, with diarrhea being the most common issue, followed by injuries and, less frequently, respiratory problems. 
Researchers suggested that stress likely contributed to many of these problems, although some cases of reported diarrhea may have been stress-related soft stools rather than true diarrhea. The findings indicate that transporting horses in pairs may reduce stress and improve welfare. Researchers also note that nutritional support may help maintain digestive health in horses that travel frequently.