The organization that preceded the Assassin Brotherhood in Assassin's Creed Origins was called the Hidden Ones. Bayek and Aya establish it after their conflict with the Order of the Ancients.
The Hidden Ones operate covertly and adopt principles that later define the Assassin Brotherhood. Their early activities take place in the first century BCE, during the political struggles of Ptolemaic Egypt and the expansion of Roman influence.
Origins presents the Hidden Ones as an early form of the organization rather than using the familiar Brotherhood name throughout the main story. The series later shows how the group develops into a more formal Assassin network.
The Order of the Ancients is their principal opposing organization in Origins. It is not the predecessor of the Assassins; instead, it is an early form of the Templar Order. The distinction is important because both groups are introduced in the same game.