Historically, the gradual transormation a Chao takes in adulthood was known as it’s second evolution. Although this terminology became embedded into Chao vocabulary, it was never actually a very good description. The Chao’s shape and colour are not permanent and will continue to change as it grows older. The key thing to note is that the Chao does not enter a cocoon, so we refrain from calling this another “evolution”.
Remember, you can always change the second evolution of a Chao by simply giving it different animals or Chaos Drives. The changes are determined by pairs of stats, as per the below diagram.
The format used in the Chao community to describe evolutions is Alignment / Evolution Type / Secondary Type.