This is the first time I've repainted the horse, so how can I improve the lighting?? Also, how do I make the tail better? You'll have to open the image in a new tab to see the detail.

Hair in general, you don't have to make the strands go all the way from the top to the bottom. You want to play with the strands' lengths and make them different. This makes it more realistic. Feel free to check out some actual photos for reference. Lighting; I personally use layers on overlay with white airbrush for highlights.

^that makes sense. Thank you!

My tips for hair:

-Make sure to have varying thickness and shades of hair strands.

-Add highlights to the hair with an air brush or blurred hard brush. For this I typically use white, set the blending to soft light, and adjust the opacity. Though its up to you how you do this

For the lighting, since its coming from the other side of the horse, I suggest adding a highlight to the edges of the body. Its hard to explain, but I feel this picture shows it well enough.
Backlight Example

For basically any form of lighting, you want to add highlights where the rays of light are hitting the body

I would use backlight as lunar explained, maybe on the shoulder areas and a little bit on the neck. The repainting/painting is amazing! Awesome job it's fantastic :D

For the hair, I would suggest not going top to bottom, but playing with different lengths, and decreasing the brush pixel size with lighter colors :D