This stamp is an derivative of an old Chinese seal. The image of the horse is a directly from the seal. The seal is pressed against white paper, using a red seal paste as the coloring material. Each time it's done, there are delicate and subtle differences resulting from the process, thus make the image unique from each time.
This horse is a symbol of Chinese Zodiac. Horse is the 7th in the 12-year cycle. Together with Tiger and Dog, they consisted the third trine. These three signs seek a true lover, and are like-minded in their pursuit of humanitarian causes. Idealistic and impulsive, the Tiger, Horse and Dog follow the beat of their own drum. Defiant against injustice, these three signs wilt without large amounts of physical affection and loyal support for causes. They are productive, enthusiastic, independent, engaging, dynamic and honorable. They can also be rash, rebellious, quarrelsome, hot-headed, reckless, anxious, moody, disagreeable, and stubborn. Although these three signs are loyal, they can be very protective when lied to.
Background of Chinese Seals can be found here : http://en.wikipedia.org/wiki/Seal_(East_Asia)
Background of Chinese Zodiac, can be found here: