not that you're lacking compelling evidence, but the helmet/face area does resemble a fox's head, especially with the top part that resembles ears.
No sheeeet, The biju weren't even seperated back then. And pretty sure the Kyubi/ other biju would have mentioned him.
There was only the juubi back then
not true. Hagoromo stated that he separated the bijuu and named the parts after having been inspired by Ashura's belief in cooperating with others
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In fact, the stream of events places that occurrence prior to the beginning of the war between Ashura and Indra so it's not entirely out of the question yet that Ashura could have very well have been its very first jin.
I'm pretty positive the bijuu's would've mentioned ashura to naruto by now....
Mito and Kushina weren't brought up as being Kurama's jin until ch. 500, and there was no point before then where Kurama mentioned it had any prior to Naruto.
I can't anything else than Ashura being way stronger than Indra.
Hashirama was able to defeat Madara when he had Susanoo + Kyuubi. And Hashirama had nothing but his own chakra. So I believe that things have always been like that. Ashura > Indra.
before Ashura managed to equal Indra, the latter was always better. Even given that Ashura eventually won the war, it was more likely due to the fact that he had backup and Indra didn't.