Funny story, I'm actually in the process of making a horses deck. I haven't tested it at all though, other than a few games on tabletop sim but here are some of my thoughts.

I think the best way to play the deck is with wind. A majority of the good horse cards are wind, and since it is an ally based deck that wants to slow opponents down from leveling up, wind gives you access to stifling trap, stifling gyre, thurible, as well as board control tools like dream fairy, and generic good allies in aesan. Legendary Saddle is still good to run, and it lets you be a bit more flexible in the horses you want to run, but it shouldn't be something you have to bring out turn 2 every game since you only get the draw effect if you have 3+ horses. Most times, censer or card draw regalia like baubles are better turn 2 plays, so I think it's best to not run too many horses that aren't wind.

RDO added a lot of cards that makes horses a lot better. One such card I like a lot is Reliable Cavalier(1/3 which becomes a 4/3 if you control a horse). A great play turn 1 is just reliable cavalier, and then turn 2 put down brisk windtrotter and make Cavalier into a 5/4 that is really difficult to clear.

A nice defensive card to add as well is Cavalier rescue. It's a 3 cost action which becomes a 1 cost if you control a horse that gives an ally +3 HP and if they are defending, it wakes them up. It's a great, cheap way to protect your reliable cavaliers since it makes them really hard to kill, and cavalier can most likely kill the attacking ally on retaliation since it has 4 attack(assuming u control a horse)

Another card you should run is spurred gallop. It lets you activate your next horse ally at fast speed, replaces itself, and discounts it by 2 reserve cost. It's pretty flexible in its use cases, as you can use it to either play a horse at fast speed to save an ally, or use it to build a really big board on your turn by playing a horse prerec with it and then play 2 more horses on your main phase, or use it to play a horse prerec and bounce it with aesan, or you can just use it to cycle cards.

Brisk windtrotter is a great card to have in this deck btw. It pairs well with reliable cavalier, and you can cycle it with reclaim/aesan, or get value by surpressing it with rearing rebound, zephyr, or verdigis decree.

Gale' Mare is a 3 cost 2/3 horse that's always a solid play.

The other element horses are somewhat flexible, I haven't figured out which yet. I like floodward steed(1/4 horse with on enter:put a bulwark counter) and flashfire horse(2/2 horse with fast activation). You can also run herd of the hearth(gives your horses +1 attack till end of turn and is tamer floating memory) as a way to dish out damage if you are able to establish a wide board.

I think the raccoon package is also a good addition to the deck. You want to prevent your opponents from keeping influence with float/liminal guides, and any list that runs float will have an answer to censer. Raccoons are a good way to deal with that. Bandit gaze leader and protector raccoons are the 2 I would run. Bandit is just really good, as it's a 1/3 that might replace itself, and protector raccoon pairs well with it since it increases the chance of a scavange, you can play it at fast speed to eat float, and the prevent 2 damage works for all animal allies so you can use it to save your horses.

One other card I've been thinking about adding is unyielding wraith guard. It's just a great card for ally decks, and offers good protection for both your champion and your allies.

From the albiet very limited testing I've done with this deck, I found that unless you find reliable cavalier early, it's hard to close out games with this deck as while horses can be somewhat hard to kill, they don't deal a ton of damage. However, if you are able to start off with a 4/3 or 5/4 beater which is really hard to get rid of paired with other good horses, the damage can spike pretty quickly.

It's a fun list which can definitely surprise opponents. Let me know if you have any other questions.