Raikou being Ushi Gozen was kind of out of nowhere, but beyond that the connections "exist" though they're tennuous. Ushi Gozen = Ox King = Susanoo =* Sakra = Indra, with that * part being where the ties get questionable. It's not something Nasu made up though. I'm assuming he made that connection by the same means through which he connected Amaterasu to Vairocana.