I just use regular shampoo on my horse. It's a lot cheaper ($1 a bottle), and it gets her just as clean as the expensive horse shampoo that's like $15 a bottle.
Here's what I do:
1) Wet her thoroughly.
2) Put the shampoo on (I usually use about a quarter to a third of a bottle every time I bathe my horse, but then again she's a draft...).
3) Use a currycomb to scrub her and rub the shampoo in.
4) Rinse her off.
5) Sweat scrape her.
6) Rinse her off again.
7) Sweat scrape her again.
I do that to make sure I get all of the shampoo and dirt off.
My boyfriend does the same thing. He drives a clydesdale and uses regular shampoo on his feathers, but he scrubs them and brushes them all the the time. There's another lady who drives his horse's brother, and she uses expensive horse shampoo, but all she does is pour it on and rinse it off, so her horse's feathers are always nasty and dirty.