I came across Rig Veda (VII.20.5), while going through a book on Vedic Literature. As stated in the question, HYMN XX of 7th Mandala was dedicated to Indra. However, going by the phrases used, ie., Bull begat the Bull, He who is Chief of men, their armies' Leader, I thought there must be some link Indra and Subrahmanya (Skanda).
I had discussed this issue with my friend, who is a Vedic scholar. He agreed to my presumption, and stated that there were some indirect references in Rig veda and Satapatha Brahmana.
The Subrahmanyâ litany, which refers to Indra, calls 'Subrahmanyôm! Subrahmanyôm! Subrahmanyôm!'. So Indra was called as Subrahmanya.
Rig veda 1.164.46 says the one Brahman is called by many names, i.e,, Agni, Indra, etc.
They call him Indra, Mitra, Varuṇa, Agni, and he is heavenly
nobly-winged Garutmān. To what is One, sages give many a title they call it Agni, Yama, Mātariśvan.
- Rig veda 1.65.1 & 2 speaks about Agni.
ONE-MINDED, wise, they tracked thee like a thief lurking in dark cave with a stolen cow: Thee claiming worship, bearing it to Gods: there nigh to thee sate all the Holy Ones.
The Gods approached the ways of holy Law; there was a gathering vast as heaven itself. The waters feed with praise the growing Babe, born nobly in the womb, the seat of Law.
Vedacharya David Frawley explains the meaning of above 2 mantras as follows:
In the state of Self-realisation, Agni, or the individual soul,
becomes the Sun or the Supreme Soul. There are certain aspects of
Agni that are most involved in this process of Self-inquiry.
Guha is the form of Agni that dwells in the secret cave of the heart
where the Self is realized. Kumara is the child form of Agni that
represents our spiritual rebirth as a realised soul. Vaishvanara is
the cosmic form of Agni identified with the Sun, but also indicates
the liberated soul. Agni's Vaishvana form reflects the process of
The inner search is metaphorically styled in the Vedas as a search for
cows (gaveshana) hidden in the cave, where they have been stolen by
the powers of darkness. This occurs relative to the hymns of Indra,
but also at times in the hymns to Agni and other deities.
The vedic cow or Go is a symbol of light, knowledge, and the sould.
It is not be literally regarded as a mere cow. The cave is the cavern
of the heart.
This epithet of Kumara in the above Rig Vedic mantra was later developed into a story. The story of the birth of Kaartikeya or Skanda was described in 223-224 sections of Vana Parva of Mahabharata. There it was mentioned that Skanda is the son of Agni.
The Rig Veda VIII.1.25 describes Indra's chariot having Peacock tails. In Puranas, Kaartikeya was described to be having Peacock as his vehicle.
Yoked to thy chariot wrought of gold, may thy two Bays with peacock
tails, Convey thee hither, Steeds with their white backs, to quaff sweet juice that makes us eloquent.
Thus, I am inferring that Indra and Skanda/Subrahmanya might be referring to one person only.