http://en.wikipedia.org/wiki/Ashvamedha
The Ashvamedha (Sanskrit: अश्वमेध aśvamedhá; "horse sacrifice") was one of the most important royal rituals of Vedic religion, described in detail in the Yajurveda (TS 7.1-5, VSM 2225[1] and the pertaining commentary in the Shatapatha Brahmana ŚBM 13.15). The Rigveda does have descriptions of horse sacrifice, notably in hymns RV 1.162-163 (which are themselves known as aśvamedha), but does not allude to the full ritual according to the Yajurveda.