When you've managed to make a chaos chao, it doesn't further
evolve.
Chaos chaos don't die with age, but they also only evolve
once.
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Chaos die at age 5 but the better you treat it the longer it
lives and it will be reborn. If you treat it badly it dies quickly
without a trace.
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Reincarnate your Chao at least twice, and give it one of all 21
animals. Don't give it any Chaos Drives!
I would say the fastest way to get a Chaos Chao is to just spend
a lot of time with your Chao so it can age and reincarnate.
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Chaos is a Water-ish type of creature who came from the Master
Emerald, also if you feed him a Chaos Emerald, he will turn more
powerful: Chaos 0, Chaos 2, Chaos 4,Chaos 6 & Perfect
Chaos.