Going back to the idea of the white horse... Sarah first sees it (in terms of narrative chronology) just before Maddy's death as she's crawling down the front steps and passing out. Leland is standing just a few feet away and doesn't seem to mind that she's passed out. So, as others here have mentioned, he's pretty clearly drugged her. "Later," in FWWM, Lynch reinstates the horse as a trope of a drugged state prefacing an attack, or at least the victimization of Sarah's family.
If we run with the idea of the white horse/death symbolism, it seems like it's Sarah's manifestation of what she perceives, albeit in a drugged, perhaps partially psychic, state. She's on the edge of unconsciousness, perhaps more susceptible to "sending" forces.
I don't know if I'm arriving at any answers, but basically I think the white horse is Sarah's own manifestation. Perhaps it would have a personal significance to her, but as the viewer isn't privy to that information, it seems that it can only represent a general feeling of dread, foreboding, and unease. I think, then, that there is no answer we can arrive at; the most we can do is recognize the use of motifs and extrapolate by comparison.
I will say, though, that it would seem a little silly to me (and quite hard handed) if the horse was thought to be a literal reference to heroin; in addition to the banality of the word play this connection would constitute, this doesn't work for me because Leland delivers the drug (as seen in FWWM) via a glass of milk. I think ingesting heroin would kill you, especially if you did it somewhat regularly as the Leland/Sarah exchange in FWWM implies. But now I'm being far too concrete and have destroyed any semblance of analysis. My bad! :)