SO. You're one of like two people who asked for this guide. Well, here. The guide you asked for. I have four images for you. You're welcome.

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Your first step is going to be selecting the correct brush and modifying it.

The brush I use for the purpose of hair is Round Brush (Angle), but the default settings don't work great so we're going to modify it a little.

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Select the brush, and in its settings go to "Shape" and turn spacing down to around 2%. This is so that it doesn't look like you're leaving a trail of dots.

Then we're going to finally start working on the tail. What I do is use the dropper to pick a very dark shade from whatever piece of hair I've left on my horse, and use that as the base- which I start on a new layer.

I strongly recommend using a brush size of at least 10px so that the hairs are defined, but cap the size at around 20-30 depending on the size of your image. You should use multiple different sizes and make sure you're not just making straight lines- curving them so that the hair looks like it's moving and natural helps tons.

Next, what I'll do is start another layer. Do NOT clip this layer on top of the first- using multiple layers with some freedom helps give the tail volume and makes sure it blends somewhat naturally with the rest of the horse.

Use the dropper to select a slightly lighter color, or choose one yourself- and then reduce the opacity to 30-50%. You may need to go over the tail multiple times to give it a little variation.

And then we do highlights! There are two ways I usually do this and I went with the lazier one on this piece. One option is you take a very pale color from your bg and make a layer with blend mode set to add, and use a brush with 3% opacity, OR you can just take a color significantly lighter than the tail and turn the opacity down to 10ish% and use that. Pick a few areas where you think the light hits the tail more and focus on those, but make sure to add milder highlights elsewhere.

That's it! You're done! I can't help you now :D

I refuse to crop. you can't make me

This is an excellent guide IMO.
I personally use Round Brush (real) for my hair, along with a custom brush I downloaded.
I would recommend toying around with force fade, and using a 1.5-5.5 size in the Round Brush, draw some fly-away hairs on your base color layer. It really helps give the hair movement and volume!

Why thank you. I'm not sure why I started using round brush angle but I'm used to it now and no one can stop me