Equinox was born in 2019. He is by Kitasan Black out of Chateau Blanche by King Halo.
Equinox was awarded Horse of the Year in Japan in 2022 and 2023, Best Three-Year-Old Colt in Japan in 2022, and Best Older Horse in Japan in 2023. Officially rated 135, Equinox became the highest-rated horse ever trained in Japan and was crowned the world's best horse in the Longine's World's Best Racehorse Rankings in 2023. He began a six-race winning streak in Grade/Group 1 company in the Grade 1 Tenno Sho (autumn) at 3 before following up in the Grade 1 Arima Kinen (Grand Prix). He kicked off his four-year-old campaign with a dominant victory by 3 1/2 lengths over Irish Derby winner Westover in the Group 1 Dubai Sheema Classic and went unbeaten through the year, winning the Grade 1 Takarazuka Kinen; the Grade 1 Tenno Sho (autumn), in which he set a Japanese record time of 1:55.2 over 2000 meters on turf at Tokyo; and the Grade 1 Japan Cup, where he defeated Japanese Fillies' Triple Crown winner Liberty Island by 4 lengths.
Equinox entered stud in Japan in 2024. His first foals are yearlings of 2026.
Equinox was inducted into the Japan Racing Association Racing Hall of Fame in 2025.