jenkins1525 is right, shampoo for a horse's mane and tail does stimulate growth. However, (isn't there always an however) each individual's hair growth is different. My sister ''hates'' me because my hair grows faster than hers. Be patient, it'll grow.
just wash it every day and eat healthy food. it will grow, but probably not as fast as you want. i've been trying to grow out my hair for the last four years and it's just barely past my shoulders right now.