If you want to keep tail and manes nice? Never ever brush them. It pulls out too much hair. Great if you want to thin it.
As for thickening?
Use White Mink, found in human hair sections, above hair dye usually, get darkest color if you need black, but if others, can match somewhat.
Wash tail, then use this.
To dry, take electric hair dryer, and dry tail, while you finger pick it, one small section at a time. And raise each piece up and let it float back down, over and over to make sure hairs separate. To bobble, take end of tails, maybe 6 inches from ends, and take it and bobble it to hair at end of tailbone very gently. Make sure it stays fluffed out and don't smooth it down.
Unbobble and then fluff by taking end and shaking it lightly to get it aired apart.
Horses make me a better person.