I knew there was a white stallion associated with rising the sun somewhere in Indic lore! Finally managed to track him down (despite the fact that he's been more or less staring me in the face for a while now):

It's Hayagriva and his Vedic prototype Dadhikrāvan!

Sridhara Babu - Hayagriva. The Horse-Headed Deity in Indian Culture

One of the several hymns dedicated to the cosmic stallion Dadhikrāvan who ushers in the morning:

Rig Veda Hymn 4:39 Dadhikrās. (Griffith Translation)

1. Now give we praise to Dadhikrās the rapid, and mention in our laud the Earth and Heaven.
May the Dawns flushing move me to exertion, and bear me safely over every trouble.
2 I praise the mighty Steed who fills my spirit, the Stallion Dadhikrāvan rich in bounties,
Whom, swift of foot and shining bright as Agni, ye, Varuṇa and Mitra, gave to Pūrus.
3 Him who hath honoured, when the flame is kindled at break of dawn, the Courser Dadhikrāvan,
Him, of one mind with Varuṇa and Mitra may Aditi make free from all transgression.
4 When we remember mighty Dadhikrāvan our food and strength, then the blest name of Maruts,
Varuṇa, Mitra, we invoke for welfare, and Agni, and the thunder-wielding Indra.
5 Both sides invoke him as they call on Indra when they stir forth and turn to sacrificing.
To us have Varuṇa and Mitra granted the Courser Dadhikrās, a guide for mortals.
6 So have I glorified with praise strong Dadhikrāvan, conquering Steed.
Sweet may he make our mouths; may he prolong the days we have to live.
Source: http://www.sacred-texts.com/hin/rigveda/rv04039.htm