The animal paired with the heavenly stem Jia in the first year of the Chinese 60-year cycle is the Rat.
The sexagenary cycle begins with Jia-Zi, combining the first heavenly stem, Jia, with the first earthly branch, Zi. Zi is represented by Rat, so the cycle’s opening designation is commonly translated as Wood Rat or Jia-Zi. The cycle then proceeds through systematic stem-and-branch combinations until returning to Jia-Zi after 60 years.
The ten heavenly stems and twelve earthly branches advance together, but their different lengths mean that not every possible pairing occurs. The stems also carry the five elements and yin-yang polarity, which is why Jia-Zi is associated with Yang Wood in traditional interpretation.
Rat is first in the animal sequence, but the important point here is the calendrical starting pair: Jia belongs to the heavenly stems and Zi to the earthly branches. They are different parts of the same formal system.