You must already have a Purebred Spanish. Or you can buy a Purebred Spanish horse in the sales already purebred. It is impossible to make a breed or breed two breeds of horses to make another breed in Howrse. Only the Administration can make new breeds.
The achievements have now ended on Howrse so you won't need to worry about this. That particular achievement goal needed you to be in the top 100 for two rankings - the easiest to make it on are horses cared for and popularity.