Brains were introduced in 1.14 (Village & Pillage) as an alternative to Goals and Targets, for use by the updated Villagers. Since then, all new mobs have been implemented using brains.
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Brains were introduced in 1.14 (Village & Pillage) as an alternative to Goals and Targets, for use by the updated Villagers. Since then, almost all new mobs (exception: bees) have been implemented using brains.