my YO got one for her horse
she seems settled with it
and ive just bought one as i will be starting to compete my 4 year old this year (i hope) and think if it helps keep them calm, its worth it
There was a study done into this which found that a lot of horses do travel better with a mirror if they can't have company. Just watch it is at head height though, as some horses can panic if they see another horses legs moving (in the mirror).