the pig (豬) is the twelfth of the 12-year cycle of animals which appear in chinese zodiac, in relation to the chinese calendar and system of horology, and paralleling the system of ten heavenly stems and twelve earthly branches. although the term "zodiac" (etymologically referring to a "[circle of] little animals") is used in the phrase "chinese zodiac", there is a major difference between the chinese usage and western astrology: the zodiacal animals (including the zodiacal pig) do not relate to the zodiac as the area of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the sun, the moon, and visible planets across the celestial sphere's constellations, over the course of the year.
in chinese astrology, "zodiacal" animals refer to fixed cycles of twelve animals. the same cycle of twelve is used for cycles of years and cycles of hours. in the case of years, the cycle of twelve corresponds to the twelve-year cycle of jupiter. in the case of the hours, the twelve hours represent twelve double-hours for each period of night and day. in the continuous sexagenary cycle of sixty years, every twelfth year corresponds to hai, 亥 (the twelfth of the twelve earthly branches); this re-recurring twelfth year is commonly called the year of the pig (豬年).
there are five types of pigs, named after the chinese elements. in order, they are: metal, water, wood, fire, and earth. these correspond to the heavenly stems. thus, there are five pig years in every sexegenary cycle. for example, in the year 2019, the earthly branch is the twelfth, hài, and the heavenly stem is the sixth, jǐ 己. the chinese new year in 2019 is february fifth: this corresponds with the beginning of both the sexegenary year of jǐ hài and also the zodiac year of the earth pig.
in the japanese zodiac and the tibetan zodiac, the pig is replaced by the boar. in the dai zodiac, the pig is replaced by the elephant. in the gurung zodiac, the pig is replaced by the deer.