If they are nilons halters I don't know there is much you can do. Chances are a lot of it is faded. You could throw in a couple t-shirts in the washing machince and see if it saves it enough.
If it's a leather halter, leather CPR works well tho practically leather cleaner would work well. A few good cleanings should bring it back to life.
Ok here is what I do with old faded halters that most people would throw away.....if the halters are not fraid or falling apart I go out and buy a dye cube and dye the halters to what ever you want...it works
I put mine in the washer in a pillowcase too, like Gingerrr does.
That dye stick is a great idea!
You can get rid of lots of the fraying by carefully burning off the fuzz with a flame. You have to be careful not to put the halter directly to the flame but as you get it closer you will see the little fuzzies burn off and disappear before your eyes.