I personally wouldn't use human shampoo on a horse's mane, tail, or coat, but if you absolutely have to, use baby shampoo. It's the mildest and rinses out easily. Human conditioner works alright, but I'd still prefer the equine variety. I use Sore-No-More shampoo. It smells like coconut, rinses out realllly quickly and easily, and leaves them with a gorgeous sheen. It alsos includes liniment, so it's like getting a nice, cooling massage for your horse! Cowboy Magic works well too, it's especially formulated for cold water baths. For conditoner, I use Eqyss Cloud Nine. It makes my geldings tail feel like silk!
I've spent most of my life riding horses. The rest I've just wasted.