The word "Chaos" can be referring to two vastly different things in the Sonic universe: one being a species of squishy blue creatures Sonic encounters in his travels, and the other being a powerful squishy blue creature Sonic fights in his travels. Because they have the same name, they will both be detailed in this page to avoid confusion.

Chaos

The Chaos' (the squishy blue species) name is not pronounced in a typical manner. In this case the word "Chaos" is pronounced as "Chows", and a singular entity would be referred to as a "Chao" (or "Chow"). It isn't explicitly clear where they originated or what the extent of their abilities is, but they have some unique attributes which set them apart from other entities in the Sonic universe.

One such attribute is their ability to mold and adapt their physical appears based on their skillsets. For example, a very fast Chao will look very different from a very strong Chao. Chaos can also learn (or forget) these skills by spending time with various animals, after which they will sometimes adapt physical attributes of said animal - for instance, a bunny's ears or a tiger's tail.

Chaos have the ability to further change their appearance based on the alignment of their caretaker. If a Dark side character tends to the Chao, it will start to shift to darker hues and, upon eventually reaching its adult transformation, will aquire a spikeball above its head in place of the customary dot. Time with a Hero character will shift the hues to lighter colors, and upon transformation will result in a halo instead of the dot.

A final example is a Chao's ability to be reincarnated. If its caretaker loved and cared for it enough during its life, then instead of dying when it grows old, it will instead reincarnate as an egg with greatly reduced stats.

Chaos

Chaos' (the squishy blue creature) name is pronounced exactly as you would expect it to be: kay-aws. In Sonic Adventure: DX, Dr. Robotnik refers to Chaos as "the god of destruction". I'm not really sure if that's accurate or what would constitute such a claim, and I am not qualified to make a judgement either way. Chaos gets stronger with each Chaos Emerald it consumes and was the final boss of Sonic Adventure: DX. Chaos appears in Sonic Adventure 2: Battle only as an additional character for the multiplayer gem hunt mode.