|Grosset & Dunlap|
The book was part of Grosset and Dunlap's Famous Horses series. Their definition of what constituted a famous horse was quite wide: although real horses do figure, there's a fair share of fictional ones.
Kentucky Derby Winner was first published by D McKay in 1949, and was illustrated by Corinne Dillon. Grosset and Dunlap published their edition in 1955.
For more on the author, see her page on my website.