The 12 zodiac signs most commonly used in 2015 were created by the ancient Greeks and Romans, and they still go by their Roman names. They were based on earlier astrological systems, which were created by the Babylonians.
The Babylonian system may have been in use as early as 2500 B.C.E. It was popular enough to spread to neighboring civilizations and may also be the basis of the Chinese and Egyptian zodiacs. The zodiac may have been based on the way seasonal changes affected newborn infants, which may have a lasting effect on health and personality.
The most popular modern zodiac, known as the tropical zodiac, was largely created by Ptolemy, a famous Greek astronomer who lived in the second century. His system left out some factors, such as equinoxes, which can affect the relative positions of the constellations. Some astrologers use the sidereal zodiac, which ascribes different dates to the traditional 12 symbols and includes an additional sign called Ophiuchus.
The 12 signs themselves are named after constellations which supposedly have the most influence on the people born during that time. The constellations are named for various Greek and Roman myths, which ancient astronomers felt the constellations represented. However, these constellations have little link to the signs due to changes over time. Astrologers also consider the role of other celestial bodies, including other planets and the sun, to create a horoscope.