I have been training horses for over a year now in BDO so I thought I share my modest 'wisdom' about it to the community. I will do it as brief as possible and mainly provide you with a quick list of do's and don't and general training tips.




Choose your lifeskill character:
First of all, before you decide to commit to the horse training lifeskill think about with what character you want to use for it. Using a character below level 50 makes training horses a lot easier, because other players can't kill you outside of safe zones.
If you use a character above level 50 then you are basically limited to training in safe zones or risk being killed while (AFK) training. This not a problem, training in safe zones is fine. However, you have to choose the right safe zone.

Safe zones:
In BDO you see a lot of horse trainers training in busy towns (especially in Heidel). This is the worst place you can choose to train your horses! Towns use up a lot more computer resources, the FPS will be lower and there are lag spikes that breaks the auto-path. Besides this there are also a lot of NPC's walking in the town using the same auto-path as you are using. Those NPC's will constantly slow down your horse. Also players tend to stand on the auto-path and in some cases not only slowing you down, but actually blocking you to a stand still. In other words, don't train in towns!

What safe zones are good? Well basically all safe zones that are quiet and have no NPC's walking on the auto-path. An excellent safe zone with a horse stable nearby is the beach near the town Olvia, named 'Termian Beach'. That beach is a very large safe zone without any obstructions. The FPS is great there, your PC will use a lot less resources and there are no lag spikes. Great spot for AFK training! Setting a good auto-path on that beach is tricky though. You want to have your horse running constantly without changing pace. The right auto-path loop on the beach is a line between the enterance of the beach (grassy area) and the shore line.
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Gear:
Always gear up your character before training. Even if it's just beginners horse training clothes (that you get through a quest), it makes a difference. I personally use the 'Fine Horse Trianer Clothes' at max level, but just get what you can afford. You also want to equip lifeskill accessories, get some Loggia accessories that you can afford and always wear them while training. On top of this you can also wear a premium 'Riding Attire' costume that will give you an extra 20% mount XP buff (quite a bit). And of course there are lifeskill pets that can be used, there is even a dog specifically made for horse training.

And very important, buy/craft a 'Manos Riding Crop'! You need a maxed out +15 Crop (at 100% durability) or you will miss out on huge buffs.

Also the horse needs some gear! You don't even have to fully gear up the horse, but you want to have a saddle and horse shoe irons, both at max level if possible. On top of that the horse also can wear a premium 'costume' which will give a slight accelleration and speed buff, but thats pretty minor.

Anything else?
No thats pretty much it as for the training part. Only thing to add is carrots in your inventory and make sure that while using auto-path the UI is set to 'auto-consume feed'. And use High Quality carrots when you train horses to max level instead of normal carrots.




Breeding:
You can have some control over the RNG odds when you breed horses. You can increase chances to get a female horse or/and a higher tier horse or a certain skin type.
This website will calculate for you what level the horses need to be at and what horses you need to get the foal you want. It's still RNG, but this website will maximize the odds to a desired result and shows the right level the horses needs to be at. https://www.somethinglovely.net/bdo/horses/

Mastery is important when want to breed horses. Ideally you want to wear 'Loggia trainer clothes' that will increase your mastery (different clothes than your training clothes) BEFORE you press the 'breed' button. Mastery will increase the odds of getting a good horse and a higher tier horse in general, but it also has some other positive RNG effects that the devs keep a secret.


What horses to train?
Female horses! The vast majority of horses are male so female horses are precious to have if you want to breed horses. Ideally you only want to train and breed T7 +T8 horses. Instead of wasting time with training lower value T5/T6 horses try to buy (snipe) breedable T7 female horses off the market instead of breeding them yourself. 1 hour of market sniping for a breedable T7 female is still better than wasting 40+ hours on training 2 lower tier horses with a 20+% chance of gaining a female T7 through breeding.


T8 Courser and T9:
This I will leave to other trainers that have experiece with this.
I know the whole process of creating a T8 courser and then go for the awakening, but I haven't done it myself yet. I only want to give tips from my own experience, not from what I've read and heard.

I only want to add this... Getting a T8 courser can 'theoretically' be done without spending real money on it, but you will need a lot of skill change coupons and you will need a good T8 horse that only needs 1 skill change to become a courser. Those coupons you can get through events and loyalty points, but this will take time before you have enough. This is just for getting 1 courser... If you are thinking that you can a become a horse trainer that produces dream horses, then forget it without spending money on coupons. It's not worth it, creating and training T9s is a lifeskill activity for the whales.



Notes:
I feel like I am forgetting a few more things that I wanted to add so I might add them at a later moment!