A Han Dynasty terracotta horse head (1st-2nd century CE).
The classic shi poetry, with its four-character lines, was revived by Han and Three Kingdoms poets, to some extent. However, among other poetic developments during the Han epoch was the development of a new form of shi poetry, dating from about the 1st century BCE, initially consisting of five-character lines, and later seven-character lines. The development of this form of shi poetry would occur in conjunction with various other phenomena related to Han poetry. It is indeed ironic that the new form of shi developed during the Han and the Jian'an period would become known as "gushi", or "ancient style poetry".
More Detail: http://en.wikipedia.org/wiki/Classical_Chinese_poetry