Chinese zodiac sign meaning SNAKE,「tail, side-to-side end-to-end」with connotations of coiling, spiralling, and snake-like sidewinding motions.「Meatus with penis」perhaps to represent the snake-like characteristics of the human penis, or more likely a SPERMATOZOON that has a shape and employs tail actions quite similar to that of a snake:
「According to an ancient Chinese text, the Chinese viewed magnified specimens through a lens at the end of a tube, which tube was filled with varying levels of water according to the degree of magnification they wished to achieve. Ingenious, effective and repeatable in the home, today. That this occurred some 4,000 years ago in the Chow-Foo dynasty and more than 3,500 years before the 'father of modern microscopy' was born is quite remarkable. That these Chinese ancients achieved magnification levels of 150 times today's standard, or 100 moou, is breath-taking.」
[ https://www.microscope.com/about-microscopes-t-19.html 2011-04-23 ]
…so, the ancient Chinese knew about sperm, how they move, what they look like—little「snakes…」
Snakes and other reptiles have「branching, skilled glans」(岐) as a「penis with orifice」:
「A hemipenis (plural hemipenes) is one of a pair of intromittent organs of male squamates (snakes, lizards and worm lizards). Hemipenes are usually held inverted, within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. Only one is used at a time, and evidence indicates males alternate use between copulations as a behavioral means of increasing sperm transfer when the interval between copulations is relatively brief. The hemipenis itself has a variety of shapes, depending on species. Often, the hemipenis bears spines or hooks, in order to anchor the male within the female. Some species even have forked hemipenes (each hemipenis has two tips).」
[ https://en.wikipedia.org/wiki/Hemipenis 2014-06-23 ]
(1) 子 rat (children, spermatozoa)
(2) 丑 cattle (herded meat assets, harem)
(3) 寅 tiger (religious dogma, enslavement)
(4) 卯 rabbit (female, reproduction)
(5) 辰 dragon (power sex)
(6) 巳 snake (fornication)
(7) 午 horse (autoeroticism)
(8) 未 sheep (zooerastia)
(9) 申 monkey (deities, anal-centric)
(10) 酉 cock (cognitive processing impairment)
(11) 戌 dog (male, domination)
(12) 亥 swine (humanity in general).

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: 6th earthly branch: 9-11 a.m., 4th solar month (5th May-5th June), year of the Snake
Japanese EDRDG: sign of the snake or serpent, 9-11AM, sixth sign of Chinese zodiac
Unihan extended: the hours from 9 to 11; 6th terrestrial branch