Animal tattoos are in vogue, not just today, but for centuries. Earlier when tattoos were based only on religious symbols or things that occurred naturally like water, trees, flowers, etc. animal tattoos had a special place. Man has been attached to animals for centuries.
Horse tattoos have long been a symbol of strength, grace and stamina, not to mention the fact that horses have only been a tamed species for a rather short period of ten thousand years; for that they are also an emblem of untamed beauty.