Cao Zhi 曹植 (son of Cao Cao 曹操, younger brother of Cao Pi 曹丕)
I must confess to finding little about Zi Xing. All that my research has thrown up is it is of the Da Yuan 大宛 breed of horses and is fast as lightning. The word 骍 is already extinct in the common chinese dictionary. It refers to a cross-breed between a bull and a horse with a reddish body. However it is pretty clear that Zi Xing is but a name borrowing on the strength of the Xing breed, when it truly is a Da Yuan breed.
Zi Xing belonged to Cao Zhi, the drunken poet famous for his "7 Step Poem 七步诗". The life and struggles of Cao Zhi can be made for a dramatic novel all by itself. The pinnacle of Cao Zhi's melodramatic life came when Cao Pi wanted Cao Zhi executed for being too drunk to attend their father Cao Cao's funeral. Cao Zhi was challenged to come up with a poem within 7 walking steps or die. That was when Cao Zhi cited his 7 Step Poem, that moved Cao Pi to tears.
Since this write-up is not about Cao Zhi, you can read more about him here on this Wiki link.