This is Feel Alright, AKA Deep Impact with the serial numbers filed off.
He's the best friend of Bunta/Hinode Makibaoo (the protagonist of Taiyou no Makibaoo). Bunta himself is based on Haru Urara. Or Haru Urara's life story, at least.
The manga (itself a sequel of Midori no Makibaoo) was inspired by the real-life Haru Urara boom at the time, and Feel Alright/Deep Impact was included because of all the Haru Urara/Deep Impact shipping back then. Though Haru Urara got genderbent in the process and became a grey/white horse.
Somewhat confusingly, Bunta's father in this series is Tamabu Cross, a clear allusion to Tamamo Cross. But Bunta's maternal uncle is Midori Makibaoo (the previous story's MC), who's also based on Tamamo Cross.
Most of the Taiyou no Makibaoo characters are also composites of real-life horses to some extent (unlike the Midori no Makibaoo ones which are solely based on single horses). There's Femme Fatale, who's based on Daiwa Scarlet but has Vodka's accomplishments instead of Scarlet's. Feel Alright himself also has some traits of Silence Suzuka (broke his leg while cornering) and Symboli Rudolf (signature cutting-in move as part of his "Betweener" pacing strategy).