Agni in Rig-Veda and Aswapathy in Savitri

अग्ने तव श्रवो वयो महि भ्राजन्ते अर्चयो विभावसो ।
बृहद्भानो शवसा वाजमुक्थ्यं दधासि दाशुषे कवे ॥१॥

O Fire, thy inspiration and thy growth and thy lights blaze in their greatness, O thou who shinest out with thy lustres; O great luminousness, O seer, thou foundest by thy strength for the giver a plenitude of utterance.

(Hymns to the Mystic fire)

A Seer was born, a shining Guest of Time.

For him mind’s limiting firmament ceased above,
In the griffin forefront of the Night and Day
A gap was rent in the all-concealing vault;
The conscious ends of being went rolling back:
The landmarks of the little person fell,
The island ego joined its continent:
Overpassed was this world of rigid limiting forms:
Life’s barriers opened into the Unknown.

(Savitri)

Sri Aurobindo's comments go:

"Night and Dawn are the two unlike mothers who jointly give birth to Agni, Night, the avyakta, unmanifest state of knowledge and being, the power of Avidya, Dawn, the vyakta, manifest state of knowledge and being, the power of Vidya.

They are the two Dawns, the two agencies which prepare the manifestation of God in us, Night fostering Agni in secret on the activities of Avidya, the activities of unillumined mind, life and body by which the god in us grows out of matter towards spirit, out of earth up to heaven, Dawn manifesting him again, more and more, until he is ready here for his continuous, pure and perfect activity. When this point of our journey towards perfection is reached he is born, śveta vāji ['white horse'] in the van of the days. We have here one of those great Vedic figures with a double sense in which the Rishis at once revealed and concealed their high knowledge, revealed it to the Aryan mind, concealed it from the un-Aryan. Agni is the white horse which appears galloping in front of the days, - the same image is used with a similar Vedantic sense in the opening verse of the Brihadaranyaka Upanishad; but the horse here is not, as in the Upanishad, aśva, the horse of vital and material being in the state of life-force, but vāji, the horse of Being generally, Being manifested in substance whether of mind, life, body or idea or the three higher streams proper to our spiritual being.

Agni therefore manifests as the fullness, the infinity, the brhat of all this sevenfold substantial being that is the world we are, but white, the colour of illumined purity.

He manifests therefore at this stage primarily as that mighty wideness, purity and illumination of our being which is the true basis of the complete and unassailable siddhi in the yoga, the only basis on which right knowledge, right thinking, right living, right enjoyment can be firmly, vastly and perpetually seated. He appears therefore in the van of the days, a great increasing state of illumined force and being, - for that is the image of ahan, - which are the eternal future of the mortal when he has attained immortality...”

We have here the picture of a divine Presence that establishes itself in the human consciousness, turning that consciousness into a figure of divinity. We have pointers to the light of a Seer-Guest being "born victorious" in one who was so far a mental creature. As a result, "fullness", "infinity", the vastness (brhat) of the spiritual existence are realised.

And by the help of this light the realised "freedoms, powers, illuminations and widenesses" are guarded. Also, the light comes from both Night and Day and moves "in front of the days" in the symbolic shape of a supernatural animal - a White Horse of illumined power - by whom or in whom our Night and Day are transcended and a greater lustre of knowledge revealed beyond them.

(An excerpt from “On Sri Aurobindo’s Savitri” by Amal Kiran, Sri Aurobindo Ashram Trust, Puducherry)