What they said (although personally I use showsheen or other horse-grooming detangler, it isn't that expensive and is better for them to get all over their skin (which it will) than WD40. Plus I need to save my WD40 to use on practically everything ELSE in my life LOL)
Just posting to add that, if you use a comb rather than your fingers, only use the end coupla teeth, you are only trying to grab a few hairs at a time and tease them out of the edge of the matt. At first it will go slowly but as you get more and more unravelled it will go more rapidly.
Also, check when the horse was last wormed, she may have pinworms which cause tail-rubbing which can cause matts; and check that she is not all crusty in the udder region (be careful, some mares are touchy about that) which can do the same.
Good luck, have fun,
Pat