My absolutely favourite product for detangling manes & tails is Absorbine's Show Sheen gel. It's a not as heavy and sticky as hair gel, but it's a bit thicker than baby oil. It's nice because, unlike the spray, you can get it exactly where you want it and it doesn't end up on the coat instead.
Usually I'll groom my horses and then put some Show Sheen gel in the tail & mane. Which ever is worse for knots & tangles is the one I brush/comb through last. In the case of today's model, Voodoo, his mane was horrible. It was full of lots of witches knots and almost dread like twists:
When I was done his tail I went and picked through his mane, some with my fingers and some with the wide tooth part of my comb. Finally I combed through with the fine side and his mane was tangle free, smooth & shiny: