You're here because you want to understand the Howrse economy a bit better. I would explain it better had I the time, but instead I'll just give you tips. Ah, deal with it.
As an introduction I'd like to discuss a major aspect of the Howrse economy. Howrse has an exponential (or near enough) growth of horses. The pipeline is such that more horses are born than are removed from the game, and as more players join a server more horses are created.
In terms of finding ways to make profit, you have to understand that most players on the game are young, naive, and outright stupid in many cases.
Rare Coats can make you a lot of money. Especially via the auctions, you can find horses sold for much less than they're worth, because sellers don't realize the rarity of coats. I consider anything 5% rarity or less to be valuable. It's almost never worth selling rare coats (non GA of course) for passes, as in my experience not even 1% coats sell for passes unless the horse is valuable for other reasons such as its purebred nature, breed rarity, high GP, or BMI attachments. Usually you can sell 5% coats for 3k - 4k equus, 4% for 4k - 5k, 3% for 5k - 6k, etc. 1% coats often fetch 10,000, but I haven't often gone higher to test this.
Pass Horses can be sold for anywhere from 70,000 to 150,000 equus, and by this I mean any non immortal horse that is over 25 (preferably just over 30). Save them up - they sell quickly and can be a great emergency fund. From natural aging, it takes 6 months for a foal to reach proper pass horse age. AP farmers probably generate a few pass horses.
Sales Flipping is a term that simply means buying horses that are cheap in sales/auctions and reselling them at a price that will still sell. For example, on International, I buy as many auctions as I can, because those damn VIPs don't splurge all over them with their autobuy. Anyway, if you buy auctions at around 1000 equus or less, you can sell those horses in the direct sales for about 1,700 equus (on International at least), and they will sell fast. Check the coats for rarity though.
Mass Covering is a term where you cover every female you own with all the highest GP males that you own. Don't use any public coverings. Give all of your equus to a friend when you do the covering, but keep an amount of equus just under 10,000, so that the coverings will only cost 100 equus in vet fees. Then, naturally age all of the covered females. When they give birth, you'll have a tonne of foals that only cost you 100 equus each, and you can sell them on the direct sales for ~1,700 or whatever you want. Again, check for rare coats.