This chapter of the Shatapatha Brahmana of the Yajur Veda describes the Subrahmanya hymn, which despite its name has no connection to Shiva's son. It's a hymn recited in the beginning of Yagnas in order to invite Indra to the ritual. Here's how it addresses Indra:
- Thereupon he recites the Subrahmanyâ litany. Even as one would say to those for whom he intends to prepare a meal, 'On such and such a day I will prepare a meal for you;' so does he thereby announce the sacrifice to the gods. 'Subrahmanyôm! Subrahmanyôm! Subrahmanyôm!' thus he calls, for the Brahman indeed moves the gods onward. Thrice he says it, because the sacrifice is threefold. 17.'Come, O Indra!' Indra is the deity of the sacrifice: therefore he says, 'Come, O Indra!' 'Come, O lord of the bay steeds! Ram of Medhâtithi! Wife of Vrishanasva! Bestriding buffalo! Lover of Ahalyâ!' Thereby he wishes him joy in those affairs of his.
- 'O Kausika, Brahman, thou who callest thee Gautama.' Just so has this (formula) been devised in these days by Âruni, to wit, 'thou who callest thee Gautama:' he may say it, if he choose, and if he does not choose, he need not attend to it. 'In so and so many days, to the Soma-feast,' (stating) in how many days from hence the pressing is to be.
Most of these references are understandable, for instance "lover of Ahalya" and "Gautama" are references to Indra having an affair with Ahalya while disguised as her husband Gautama as I discuss here. And "wife of Vrishnasva" is a reference to the fact that he had an affair with the sage Vrishnasva's daughter while disguised as the girl's mother. But I'm interested in the fact that Indra is addressed as "Kaushika".
For those who don't know, Kaushika is a term for the descendants of the ancient lunar dynasty king Kusha. The most famous Kaushikas are the sage Vishwamitra and his descendants. In any case, this excerpt from the Jaiminiya Brahmana of the Sama Veda provides an expansion for why Indra is addressed as Kaushika:
"O Brahman Kaushika, who callest thyself Kaushika." Verily when he got into a mighty encounter with the Asuras, at that time he put the Vedas away. He learnt them, indeed, of Vishwamitra. That is why he called himself Kaushika.
My question is, what is the story of Indra relearning the Vedas from Vishwamitra? Did his battle with the Asuras wipe his memory of the Vedas, or did he make himself forget the Vedas so that he could fight ruthlessly against the Asuras, or what? Could this be a reference to his incarnation as Indra Vaikuntha, in which as I discuss here he lost memory of who he was? And in any case, why did he relearn the Vedas from Vishwamitra of all people?
By the way, I should mention that the commentator Sayana interprets the use of Kaushika in a different way, namely that it's a reference to Indra's incarnation as Vishwamitra's father Gadhi. Now obviously I would trust the Jaiminiya Brahmana over Sayana's commentary, but I suppose both could be true: Gadhi could have asked his son Vishwamitra to reteach him the Vedas!