Verse 13.4.2.2

tadyatsarvarūpo bhavati sarvaṃ vai rūpaṃ sarvamaśvamedhaḥ sarvasyāptyai sarvasyāvaruddhyā atha yajjavasamṛddho vīryaṃ vai javo vīryasyāptyai vīryasyāvaruddhyā atha yatsahasrārhaḥ sarvaṃ vai sahasraṃ sarvamaśvamedhaḥ sarvasyāptyai sarvasyāvaruddhyā atha yatpūrvya eṣa vā aparimitaṃ vīryamabhivardhate yatpūrvyo'parimitasyaiva vīryasyāvaruddhyā atha yaddakṣiṇāyāṃ dhuryapartidhura eṣa vā eṣa ya eṣa tapati na vā etaṃ kaścana pratipratiretasyaivāvaruddhyai

Preview of English translation:

2. And as to its being one marked with all colours, it is for the sake of his (the Sacrificer's) obtaining and securing everything, for colour (outward appearance) is everything, and the Ashvamedha is everything. And as to its being perfect in speed, it is for the sake of his obtaining and securing vigour, for speed is vigour. And as to its being worth a thousand (cows), it is for the sake of his obtaining and securing everything, for a thousand means everything, and the Ashvamedha is everything. And as to its being in its prime, it is for the sake of his obtaining unlimited vigour, for such a one that is in the prime (of youth) increases to unlimited vigour. And as to its being without its match under the right-side yoke, it is for the sake of his obtaining yonder (sun), for that (horse) indeed is he that shines yonder, and assuredly there is no one to rival him.

For a detailled translation, including proper diacritics and footnotes, go the full English translation.