Sailor Chaos is Chaos reborn. According to Sailor Cosmos, Chaos will emerge from the Galaxy Cauldron reborn as a powerful Sailor Senshi. Sailor Cosmos, frustrated with an ongoing battle against Sailor Chaos, disguised herself as Chibichibi to travel back in time and convince Sailor Moon to destroy the Galaxy Cauldron, and thereby prevent Sailor Chaos emerging to start a new series of devastating wars.
Sailor Chaos never appears in the anime, but Chaos appears very briefly in episode 200 being expelled from Sailor Galaxia after she is defeated. Other than this, Chaos does not appear in the anime series. Sailor Galaxia ultimately reveals herself as the soldier who sealed Chaos within her own body and thus saved the galaxy. Chaos possesses Galaxia in order to give her the power to fight the kibou no hikari - the Light of Hope - Eternal Sailor Moon. But Usagi is able to reach the small bit of hope left in Galaxia's own heart, and she releases Chaos from her body, ending the anime story arc.