i have the book of that and spirits father is a black mustang and his name is strider

we assume a black kiger mustang, but he could be any colour really. try the colour calculator on google, then see how many coats go with palomino to make a buckskin foal. any that work are potenitial fathers to spirit.
i only heard of the books when spirit source started up, probably because we had little to no merchendise in the uk. maybe thats why not many people are aware of them.

In the books, his father was a Black Stallion named Strider, however it's uncertain what happened to him before Spirit was born because no black horses were shown in the herd at the time.

A lot of fans though don't recognize the book series as canon and create their own versions, seeing how the books weren't that popular anyway.

I haven't personally read the book(s) that the movie was based off of. But in the movie, there is no stallion who gives us any reason to believe that he's Spirit's father. None of the stallions played with Spirit; in fact, there looked to be no stallions at ALL in the Cimarron herd, let alone any that look like they could be Spirit's father.

So unless his father is mentioned in the books descriptively, Spirit's father is a toss-up: he could be ANY stallion in the Cimarron herd for all we know.