Veronica Cox, from the UK, wrote: "Love this product, makes you hair look fuller." Urban Outfitters' website says Mane n' Tail is "loved by humans and horses alike".
Sarah Coonan, Liberty's beauty buyer, said: "Mane n' Tail is the original horse-to-human crossover brand.
"It leaves your hair clean and shiny without stripping any natural oils. Our customers love it too, it's Liberty's best-selling shampoo."
Mane n' Tail was created by the Katzev family, who own a horse farm in New Jersey, for show ponies with "long flowing manes and tails".
Devon B. Katzev, president of the brand's parent firm Straight Arrow, said the crossover happened when horse owners started using the products on their own hair and word "spread like wildfire". He added: "Our original formula [has] been a best-kept secret for beauty aficionados everywhere."
Liberty and Urban Outfitters are selling Mane n' Tail shampoo and its conditioner for £7 (NZ$14.60) each.