Though it is not cheap, you only need a very small amount, I can recommend survivor detangler from Equss. I have a gypsy cob and it works every time on his very long and thick tail. It is also a leave in conditioner with no silicon etc to damage the hair.
Alternatively do as I do, keep it in a tailbag over winter and in the summer, I put two thirds of it into the bag and leave the top third layer out as a swisher - this is still more tail than most TBs left out.
You can use pig oil/baby oil as a detangler - cheap and very effective.
Think twice about using furniture polish if you intend to use it regularly - lots of nasty chemicals and after a while the hair looks dry and brittle. Ok for the odd show as long as you rinse the tail out afterwards.