I think that with some horses, it does. My mustang Koda has always had a very long, thick mane but it was uniform in it's thickness. Last summer, he wore a part about 5 inches long off in the middle down to the follicles. Now it is growing in much thicker than the rest of his mane.
I know that when I was a kid, my hair was ridiculously thin and stringy. Mom started trimming it about once every 2 weeks and it started growning in thicker and darker (but that may just be because I was growing up too, I don't know).
Here is another one that will trip you out (people hair though). My aunt Carla had cancer about 5 years ago. Before, she had bright red, very straight hair that went down to her ankles. After she went through chemo she lost her hair. When it grew back in, it was a light brown and wavy.