The horse sacrifice. The Ashvamedha (Sanskrit: aśvamedhá) was one of the most important royal rituals of the Hindu Vedic religion, described in detail in the Yajurveda (TS 7.1-5, VSM 22–25 and the pertaining commentary in the Shatapatha Brahmana ŚBM 13.1–5). 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.
As per Brahma Vaivarta Purana (185.180), the Ashvamedha is one of five rites forbidden in the Kali Yuga, the present age.