Here's what I have so far - if you know a mould that would better suit, please let me know!
American Standardbred: Flash
Standardbreds have a pretty wide range of build. Some look like thoroughbreds, some look like tennessee walkers, and some look like morgans. They have generally straight to slightly-roman profiles, and a medium-to-light build, which is why I chose Flash. Breyer does have a standardbred model (the Pacer) but I don't care much for the Pacing gait, nor that model.
Ardennais: Wintersong
Just like the Standardbred, the Ardennais also has a wide range of builds. Some I saw were so beefy I could probably sustain my entire family for a year off of it - if we ate horse. Which I hope we don't, but we've eaten some pretty questionable meats. Anyway, others I saw looked similar to shires and Suffolk Punches, so I went with a medium build, and chose the Othello/Wintersong mould.
Cleveland Bay: Cedric/Ideal
Breyer does have a cleveland bay mould - most commonly the Irish Diamond mould. However, when I looked at the real horses, the mould seemed too hefty for their build. They have cresty necks and a slightly roman profile, but the rest of their body looks like a thoroughbred, or a light warmblood. So I chose Cedric or Ideal. If I choose Cedric, I'm going to try to find Ideal's tail to use, since I'm not a fan of Cedric's. If I find the ideal mould, I'll have to remove the mane to add the crest, but I won't have to find the tail, so there's that (although addmittedly the ref I posted could fit the cleveland bay mould, so that's not out of question, it'll just be a last resort.)
Dark Horse: Cigar
Since RDR doesn't specify a breed for the Dark Horse, I decided to choose the meanest looking mould I could find. While Smarty Jones has an "aggressive" pose and already had his ears back, I figured Cigar's pose looked as though he were charging after something, whereas Smarty Jones just looks like he's running for his life (probably from Cigar ;)) After all, you can only get the Dark Horse if you kill so many people to bring your honour in the negatives. Sounds pretty aggressive to me! I'll probably customize it to look meaner, tough. Mainly just flatten the ears. Might lower the neck and outstretch the head some, depending on how I feel.
Dutch Warmblood: Ruffian
Dutch Warmbloods have a very thoroughbred-type build, with a straight profile and a long, medium-to-thin neck. Since I'm trying to keep forward motion the recurring theme here (since I highly doubt your horse will be standing for long in RDR unless you've hitched it and are attempting to climb the stairs to your room in the Armadillo saloon after getting wasted,) I went with Ruffian. She showcases a nice, med-light build, a straight face, and a long, medium-width neck. Alternatively, I could also use the Idocus mould, but that'll be a last resort as it's standing.
Highland: Haflinger, Gretel, or Fjord
One of my favourite pony breeds, the Highland has a very drafty build, with a wide, cresty neck. Traditional-scale breyers have a very small selection of un-gaited ponies, so I chose the only one drafty enough - the halfinger. I'm not a fan of this mould, either, and it'll need a lot of work to be made into a feasible highland. Alternatively, I could use the Henry Fjord horse, but my money is personally on the Gretel mould. I've been known to use Classics as Traditional ponies if they can pass as one, and this horse will need the littlest amount of work to - well, work. Plus, my local Meijer had them last I checked so if they for some reason seem hard to find in body form, I can check there.
Hungarian Half-Bred: Giselle
The Hungarian Half-Bred horses are pretty interesting to say the least. They're crosses between the Hungarian horse and another breed, which means that their build could change depending on the breed that they were crossed with. However, commonly, they have ridiculously dis-proportionate necks to their body. They're thin, and their bodies are generally stocky in comparison. So what model replicates that? Giselle. Her neck doesn't look that thin because of her mane giving the illusion of thickness, but if you remove that mane, you'll see that it's very thin. Plus, her facial profile matches, which is a definite added bonus, since the face is where people generally focus. In the game, there's a stranger side-mission that requires you to go catch a "White stallion" to free a chinaman from his indentured slavery (you're paying for the chinaman and then you set him free) who later falls victim of opium. At any rate, canonically after that mission, you get this idea that every half-bred that you catch is automatically a stallion. At least I did, though in my mind all of the horses are stallions - except War from Undead Nightmare. So I may make the model a stallion, depending on how much work I want to do. (You wouldn't think testicles are hard to sculpt but man, they are!)
Painted Quarter Horse: Latigo, Cody
The "Painted" Quarter Horse is my least favourite of the appaloosas. It looks pink. Not only that, but the face is generally ugly. So, I gave myself the options of the two best-looking quarter horses that aren't my favourite. I do like Harley D Zip, but I'm not a fan of his mane and tail - which I can replace or redo, which is no big deal. There's also the option of Cody, which I think is a nice model compared to some, and it can pass as a trot, or even a lope, or a transition into either one. So I may end up using Cody for the "Painted" Quarter Horse, and save Harley for the next one. Also, I didn't provide a reference of a real QH because I'm pretty sure everyone knows what a Quarter Horse looks like. If not, google's your best friend!
Quarter Horse: Smart Chic Olena
I'll admit that the Quarter Horse is one of my favourite of the average horses. Each horse in the game has a set speed - the Standardbred was the fastest before War Horse, and the Donkey was the slowest after the diseased horses. Anything that's not diseased, a donkey, or a Saddler/Hungarian/Standardbred falls into the average speed zone. Which makes no sense considering they have three standardbreds but only the black one is fast
Anyway, because the Quarter Horse is one of my favourites, I chose the - in my opinion - best-looking Quarter Horse mould that has forward motion - Smart Chic Olena. 
Kentucky Saddler: American Saddlebred Stallion
The Kentucky Saddler - Also known as the American Saddlebred (though is surprisingly rare to find in Kentucky - for sale anyway - because of their Tennessean cousins) have a very distinguishing look. They sport long bodies, equally long necks, elegant and regal heads, and carry themselves with pride. Breyer has the saddlebred stallion, which I will use for profilic (which isn't a word, but work with me here) reasons.It will need a lot of work to not break my "no standing models" rule, but I'll manage.
Painted Standarbred: ?
It took me a while to finally decide to use Flash for the Standardbred - and even still I feel like I should use Smarty Jones or something since the Standardbred is the horse I ride in-game. It's my horse of choice, actually. So with that, I haven't completely decided what horse to use for the other Standardbreds, though I think if I do end up using Smarty Jones for the black Standardbred, I'll end up using Flash for this one. (Besides, Flash would look cute as a varnish appy.) There's nothing stopping me from using one mould for various horses, and I very well could use flash for all of the standardbreds, or smarty jones for all of them, but I'll have to decide once I actually get them in-hand. I'll end up frankensteining it if I can find heads from other models that'll fit to make it match the profiles. Even if I don't use Flash for any of these, I plan to use him to make Archibaldo (Jessica's mount in Before the Storm) so getting him either way won't hurt.
Standardbred Pinto: Same as above
Tersk: Lonesome Glory
Tersks are also interesting. Some have definite arabian influence, and others look more like holsteiners. Regardless, I decided the Lonesome Glory mould would suit, although alternatively I probably could use the Weathergirl or Huck moulds.
Tobiano Pinto: Cody
Pintos are horses that have the paint markings, but no Quarter Horse or Paint blood in them. So, that said, I could literally use whatever model I want to display this horse. For simplicity's sake, though, I'm going to say I'm going to use the Cody mould until another model pops up that wants to be the colour instead. If not, then I'll just use Cody. Admittedly, the pinto isn't my favourite horse. In fact, its one of my least favourite in the game - right there with the diseased horses. It looks like my dad's maroon shirt after going through the wash for ten years. Plus, I'm generally not a fan of the horse's pinto markings. So, Cody is fine by me!
Turkmen: Marwari
The Turkomen is very similar - if not phyisically identical to the Akhal-teke. It has a light body, thin, snakey neck, and large ears with a long, narrow head. Since I do want to give each breed its own respective model, and the Lonesome Glory mould was already designated for the tersk, I decided the Marwari would suit the Turkomen. That, and it really does do a good job portraying the breed. I'll just have to redo its ears and I personally want to give it a little forward movement, but we'll see how I feel when I actually get on in-hand.
War Horse: Smarty Jones
Just like the Dark Horse, War Horse was never designated a specific breed. Unlike Dark Horse, however, War Horse isn't acquired aggressively. In fact, up until recently, you had to buy it. (I logged in once after not playing for over 6 months and the horse came up to me and all of my guns were gold and I had all of the Social Club cheats - and apparently I'm not the only one. I'm not sure what the occasion was, but it was a pretty awesome surprise) Still, I wanted to use Smarty Jones for something, and a silver dapple black-brown would be pretty awesome on that horse.
Welsh Mountain: Haflinger or Gretel, Henry Fjord
Just like the Highland, the Welsh Mountain pony is a stockier-type pony. It has similar build to the highland, but less crest. Gretel should work just fine for it, so will be my first choice.
Lusitano: Andalusian Stallion, Esprit
Lusis are one of my favourite breeds of horses. They have just enough stock while still looking elegant. Their faces are that perfect mix of straight and roman, and their gaits are perfect. Unfortunately, Breyer doesn't have a traditional-scale Lusi - they've made them on the Andalusian and Legionario moulds. So, I will, too. Admittedly, the right front leg of the andalusian bothers me, and deters from the sculpt as a whole. Once I get to this horse, I'll think of ways to make it more pleasing to the eye.
El Señor/Picor/Hedor: Brown Sunshine, Donkey
It's going to be admittedly hard to find a donkey or mule to do these. I found maybe three "affordable" Hess donkeys on ebay, but two were auctions, and I hate auctions.
UNDEAD NIGHTMARE
War: RuffianThe "first" horse of the apocalypse (though not actually - the first horse is conquest,) this horse sports a fiery mane, blood-bay coat, and battle scars. She sets the undead on fire upon impact, and serves as a source of light (and heat I'd imagine) - No idea how I'm going to do the flames, but I'm probably only going to do the colours of the mane without it.
Famine: Lonesome Glory, Marwari
My least favourite of the horses of the apocalypse - mainly because it literally does absolutely nothing but summon a constant droning of insects, Famine is "technically" the second horse of the apocalypse. Though I forgot what was actually the second horse in the bible. Anyway, emaciated, this horse has what looks like runny bird poop on its butt. That's pretty much it. But it's fast, so there's that. I figured the thinner models would be easiest to emaciate come time for customization.
Pestilence: Lonesome Glory, Marwari
One of the coolest-looking horses of the apocalypse, Pestilence is an emaciated horse with visible flesh wounds and a green-ish tint to its grey body. Just like Famine, it does absolutely nothing to the zombies, but come on - it looks awesome. Added bonus? Its eyes bleed when it stands still. Literally, blood drips onto the ground. Creation-wise, this will be very similar to my zombies, which means it won't be hard to do. plus, the blooood and those eyes<3
Death: Smarty Jones
Hands down my favourite horse in Undead Nightmare for its benefits (not counting the gay unicorn. The Gay unicorn will always be my favourite), Death leaves a mist in his wake, and explodes the heads of the undead upon impact. Seriously, if you have this game and horse and haven't done so already, take him through Jorge's Gap. YOU WILL NOT BE DISAPPOINTED. (Though you might get some lag.) He has icy blue eyes (as though fogged over) and is a silver red roan of sorts.
THE GAY UNICORN:
Okay so it's not actually called "The Gay Unicorn" but come on, anything that shoots rainbows and butterflies out of its butt can't be straight. Unfortunately, I can't think of a model to represent such a magnificent, sentient creature. So suggestions are welcome! (I thought about the silver mould, but they're hard to find in body quality)
(Or customize resins depending on if I can find an unpainted/body cheap enough haha)
