I have used human shampoo and Dawn dish soap before. I tell ya, the Dawn works awesome for white horses or white markings! If you scrub on the nasty stains it will come nearly completely out. Just be sure you wash everything out.
However, I heard from someone that you should never use human shampoo on animals as it can damage the skin, but I don't see how that could hurt them. I've been using it for a long time and never had any issues even with dry skin or anything. My horses looked gorgeous last summer.
I've spent most my life riding horses...the rest i've just wasted.