Eh, I'm of a mind that if you don't need the horse to really sparkle up close like for a show or something, I just stick to rinsing without shampoo. A pressure handle on a regular old hose will get most the obvious dirt out of their coat and you aren't washing away the natural oils from the hair.
I used to wash my gray horse with soap every year before carrying the flag in the local parade. Then, one day I was running late and didn't have time to use the soap. I just used the regular old water pressure and it got him almost as clean as using the soap. Now, the only parts of him I soap up are his tail and his lower legs to get the majority of the stain and discoloration out.
However, if you do use soap, like others have said, anything that's safe to be used on humans is safe to use on horses. The only thing you really need to worry about is making sure to get it completely rinsed off.