"guzheng" consists of two Chinese characters - "gu" and "zheng". When the instrument was first played 2500 years ago, it was known simply as "zheng". The character "gu", meaning ancient, was added much later to honor its antiquity.
So how did the zheng come about?
No one knows for sure, but theories abound.
This theory holds that one time Huangdi (Yellow Emperor, 2878 2768 BC) was entertained with music played on the se by his courtesans. As he listened he became more and more saddened by the music. He finally couldn't bear to hear it anymore and ordered the instrument destroyed. The se was split in halves, each with 25 strings. Then during the Qin Dynasty (221 206 BC), two uncultured brothers fought over a se. They ended up splitting the instrument again. This gave rise to the zheng, with 12 strings.
Zhu was an ancient percussion instrument made of bamboo and had 5 strings. There was historical account in the Eastern (Later) Han Dynasty (25 220 AD) that zheng was also made of bamboo and had 5 strings. Both instruments were cylindrical in shape with a narrow neck. Because of these similarities, some people theorize that zheng was adapted from zhu when the playing technique progressed from striking to plucking.
A history book in the Sui Dynasty (581 618 AD) stated that "zheng, 13 strings, so called sound of Qin, invented by Mengtian" . Mengtian was a general in the Qin Dynasty who was also credited with inventing the brush for Chinese calligraphy. There are doubts about this theory because Mengtian did not become famous until 221 BC and zheng was mentioned in Qins official records 16 years before that.
The likely scenario is that zheng started out as a bamboo instrument with 5 or less strings and became popular during the Warring States Period (475 221 BC). This theory appears plausible because the Chinese ideogram for "zheng" uses bamboo as its radical, and using the material it was made of as the radical was a common way of creating ideograms for Chinese musical instruments. Later on in the Qin Dynasty, it adopted the superior design of the se and increased to 12 strings to expand its tonal range and used a wooden box to achieve better resonance.