Spirit's parents: Strider (who doesn't appear is the movie, but is in some books about Spirit) and Esperanza.
Strider maybe looks a little like his son (who'll come in future^^), but, eh, in the movie also horses were very similar.
P.S. I didn't know how it's hard to paint the sunset
damn, now I KNOW!!
Strider and Esperanza = I don't own them.