If you're using GIMP, you can completely skip my tutorial and go here: https://aliyaahgrl.deviantart.com/art/Realistic-Hair-Tutorial-373769257 This is more geared towards GIMP users while my tutorial is geared towards photoshop users.

This is a two part tutorial. Part 1 will cover how to draw the hair and assumes you'll be creating a mane/tail from scratch. Part 2 will apply a lot of what`s in Part 1 to touch-ups on a pre-cut horse.

The Setup
The brush set up is the most important part and the key to drawing out hair. Once you have your canvas set up, go to your brushes, click "other dynamics" and "fade". This causes your brush strokes to gradually fade away and results in that nice whispy look. The amount of fade you use will depend heavily on what you're doing and your canvas size. In general, the smaller the number, the more fading.

http://u.cubeupload.com/Demonically/4b11.png

Next, pick one of the default round brushes. Set the hardness to 85% or thereabouts. Don't touch the opacity;that'll be for later.

http://u.cubeupload.com/Demonically/6b32.png

Drawing It Out
Pick your colors. I'm using 6 colors plus "white". I sort of go in a lazy arc towards pure white, but never use pure white or pure black. If you do, use it at a low opacity. What you’`re wanting to do is start with the darkest color and work your way up to the lightest using progressively smaller brushes. The final, lightest color will be using a 1px brush for highlights. You don`’t have to use as many layers/colors if you’`re working small since a lot of the detail will be lost. However, if you like uploading the full sized, finished image to DA like I do, going the extra mile can add that final pop to an image.

http://u.cubeupload.com/Demonically/44f3.png

Anyway, set to work blocking out the shape of the hair; here, I'm using a 10px brush. Don't be afraid to squiggle. Hair isn't pretty. It's messy, wild, and goes all over the place. Think perpetual morning hair and not “spent "3 hours at the salon"”. You'll want to keep the hair closest to the body/neck the darkest and lighter the further away it goes.

http://u.cubeupload.com/Demonically/de25.png

Once everything is nicely blocked out, pick your next color,make a new layer, and drop down to a 4 or 5px brush and bump your fade number,if needed (I went from 200 to 300). At this point, I can’t stress enough the importance of layers. It can be the difference of only having to redo a layer and drop kicking your computer because the whole thing is ruined. Colors two and tree are really where you define the shape/shadows of the hair, so be prepared to spend more time on those.

http://u.cubeupload.com/Demonically/b964.png

Already with a little bit of work, the shape of the tail is starting to come out. Don’t worry about the thick brushstrokes of the base layer. Those can be erased later. Right now, it’s just to give you a general idea of where you want to paint.

http://u.cubeupload.com/Demonically/4916.png

When you finish with color two, make a new layer, drop down a brush size, and continue on. At this point, you can start adding little details like fly away hair clumps.

http://u.cubeupload.com/Demonically/de98.png http://u.cubeupload.com/Demonically/70710.png

Once you’re about half way through your colors, grab the smudge tool. 4% strength, round brush, 0% hardness, about 10-13px. Lightly brush over the spots where you can see where you started the brush stroke. Make sure to follow the hair; your goal here is to smooth, not blur. At this point,drop the opacity of your main brush down to about 80%. Start following the more obvious highlighted areas. Lightly smudge as needed.

http://u.cubeupload.com/Demonically/58c9.png

For the final two colors, switch to a 1px brush and just selectively highlight areas. Be more liberal with your smudging, as well. Also,don’t be afraid to go back and do more hair on the lower layers. I wasn`’t happy with the above product, so I decided to go back and add more hair.

http://u.cubeupload.com/Demonically/12.png http://u.cubeupload.com/Demonically/11.png http://u.cubeupload.com/Demonically/13.png

This uses the same method as before, but applied to a horse. Same brush settings. This time, sample colors from the horse's (rather pathetic) tail. We're only going to use 4 colors this time around. Block out the shape, don't worry about being pretty.

http://u.cubeupload.com/Demonically/9e71.png

Start building up the color layers like in part 1.

http://u.cubeupload.com/Demonically/1b32.png

http://u.cubeupload.com/Demonically/6a43.png

Good 'nuff.

http://u.cubeupload.com/Demonically/7ea4.png

Do any clean up work (such as getting rid of the big hairs). Now, in order to blend it, duplicate your tail layers by ctrl + click, right click "duplicate layers". You'll have a bunch of "[Layer name] copy". Make sure those are still highlighted. Right click, merge layers. Now hide your original layers so that the only tail layer is your merged copy.

http://u.cubeupload.com/Demonically/2795.png

Grab a fuzzy round eraser, set it to about 20% opacity. Start erasing the top of the tail.

http://u.cubeupload.com/Demonically/7bf6.png

New layer. Sample colors from the horse's tail in areas close to the drawn on tail. Start painting over the new tail. If needed, do a bet of painting on the old tail to help sell it.

http://u.cubeupload.com/Demonically/abb7.png

Now, the biggest glaring issue is that the new tail is a touch lighter than the original tail. Easily fixed. Make sure your tail layer copy is selected. Ctrl + M brings up the "curves" window. See the angular line? Click on it in the middle and slowly pull it down a bit. Different areas will affect the layer differently. Experiment!

http://u.cubeupload.com/Demonically/7548.png

Just a tiny adjustment made a big change.

http://u.cubeupload.com/Demonically/4ab9.png

Yay! Thanks Eury!!