I've was raised around horses, since my mom loves them. (not a zoo to the best of my knowledge) She always says "The best way to make a small fortune with horses is to start with a large fortune!" The point simply being they're expensive! IIRC, she said you want a minimum of one acre of field space per horse. Here is a short list of regular expenses related to ownership that I remember (I haven't lived at home in about 12 years and my memory isn't the best lol!)
Straw or wood shavings for bedding depending on preference (my mom is a shavings person)
Hay as a main food source
Grain/dietary supplements
Worming medication
Fly spray
Ferrier (shoes aren't particularly necessary if you aren't going to be riding on asphalt/concreate but they still trim and clean out hooves)
manure removal... the shit piles up, literally!
TIME... a lot of it! Stalls should be mucked twice a day (morning and night). Horses should be taken out to pasture as much as possible as it's not healthy for any living thing to be constantly cooped up in a small space like a stall, but they also need to have a place out there to go into shade/out of rain.
I should also mention that as social animals, it's best that if you plan to get a horse to keep at your property, you really should get at least two. Horses are very sociable creatures and it can cause mental issues if kept in solitude. These are just the basics, and I'm sure someone will jump in to correct me/add something I missed, but I hope this helps!
p.s.- a lot of horse owners in my area board their horses. As in, they own the horse but keep it at a place nearby that takes care of almost all the basics for a monthly fee. Granted being on a zoophilia site I assume you probably want to get down and dirty with your horse so that wouldn't be ideal, but thought I'd mention it just in case!