Board Thread:Theories and Speculation/@comment-31338621-20190722212553/@comment-732150-20190723020842

It was discovered, Madara and Izuna were more a rediscovery. Going strictly with the manga, Indra could use Susanoo, meaning he had MS. If you want to get really technical, Itachi said Madara and Izuna were the first Uchiha to have MS. Indra wasn't an Uchiha, he was an Otsutsuki from whom the Uchiha descend. The wording also leaves open the possibility of other pre-Uchiha Indra descendants to have awakened it (we don't know when the name Uchiha actually came around), the knowledge of it simply being lost through generations (no doubt many clansmen getting killed contributed) until Madara and Izuna awakened it again.