Depends on how tangled it is......if it's super matted...I normally just cut out the matted area as the tail will grow back. Otherwise I've used WD40 in the past. 

Good idea.  I used my fingers but broke a nail or two.  The metal comb would have worked well, I think.

If you're going to cut it, just run your pocketknife downwards through the middle of it, then you can usually tease the rest of the witch-knot open. That way you can salvage some of the tail hair-

This is exactly what I do when the tangled tail is nothing but a tight wald and matted, I use a small steak knife and go into the top of the matted mess and go down wards, I have saved a many of tails doing this, I use Show sheen and soak the tail in it befor I start with the knife. I would have friends call me over to do their horses tails for them, they might come out with a shorter tail but to me it beats pulling out all the hair and making the tail thinner, with all the pulling I have seen people do really makes a horse have a sore tail.. Thats why you see horses start to wiggle after a while with all the pulling it hurts.. I always find a piece of wire are a stick in the middle of the wads.

I had to do that one time on Faci's tail.  Still took me hours to get it out!  But it was worth it not losing half his tail.   He had a stick/vine hidden down deep in the mess!