Sorry, should probably itemise some stuff:
We are on DIY which is £60 a month, but that is very cheap, even for DIY, as it is a farm, not a livery yard. Part livery is usually about 40-50 a week and full livery can exceed £100 a week.
Then there's hay at around £1-2.50 a bale, depending on where you are and how available hay is that year and whether you take it off the field or have it delivered etc. You may want to feed haylage but that depends on the type of horse. We will not feed haylage to ours.
Hard feed, again depends on type of horse and what you do with it. A shetland would probably live out all year, unless you wanted to keep it in, in which case it doesn't really need anything but hay and grass. Bags of HiFi are around 10, then the nuts are around 10 a bag and then you have things like cod liver oil, garlic, carrots, all that. Feed supplements.
Vets can be as little as 200 a year for the jabs, but can be higher for injuries and if you have an older pony.
Farrier varies but is around 50-60 if you have a set of shoes put on (and we also have road studs put in). Again depends where you are).
You will probably want to think about insurance; even if you don't hack it out, it could still bite someone or injure someone or get hurt itself, and all these things can mean big bills. Ours is about 300 a year per horse.
Then you may want to transport it to shows. If you don't own a lorry/trailer like us, you can hire them or pay a friend who will need to alter their own insurance to cover you.
Oh and there's the bedding if you stable the horse. That depends how wet your horse is. Mares are usually wetter, but our gelding is actually wetter than the mares. You can use rubber matting on its own, but other stable users will complain (as we found with just having matting and banks). That's about 400 for a stable.. Then there's shavings at about 6.50 a bag and you will probably use a bag a week averaged out over the year. Straw at £2 a bale you will use similar amount but it isn't as effective and greedy horses eat it. I use megazorb which is 4.85 a bag and I use half a bag a week. There's also paper and other variations of wood bedding.
There are other expenses like lessons, and we take ours away on holiday a lot - a weekend a month and then 2 weeks in Sept, and also to shows and workshops.