Sailor Chaos

Sailor Chaos was the Senshi form of Chaos, and was only shown once in the Sailor Moon manga, in Act 52. While she did not actually play a role in the regular timeline of the story, Sailor Cosmos revealed that Sailor Chaos was a very powerful force of evil in the far future, where she caused war and destruction to the point that nothing could be salvaged even if the war ended. Sailor Chaos was allowed to come into existence partly because Sailor Moon chose not to destroy Chaos and the Galaxy Cauldron when she had the chance. The result of this war was never revealed, as Sailor Cosmos left to resume her fight as the series ended.

Anime
Sailor Chaos was never shown in the anime as it was in the manga, but this term is sometimes applied by fans to the black winged form Sailor Galaxia attained when her body was completely consumed by Chaos. It is unclear if this form actually was Sailor Chaos, as she was never addressed by name for the brief time she appeared. However, on an official Sailor Moon Sailor Stars trading card printed by Amada, she was named "Chaos Galaxia."

SeraMyu
In the musicals Starlights - Ryuusei Densetsu and Kakyuu-Ouhi Kourin, Sailor Galaxia (played by Yumie Sakaguchi) ended up being totally corrupted by Chaos. When that happened she appeared in a darker version of her sailor fuku, and announced herself as Sailor Chaos.

Trivia

 * According to ancient Greek belief, Chaos represented the lack of order or predictability, and was the antithesis of Cosmos.