Although we all want the perfect horse that does everything we ask of it, does not get sick, does not take tons of our money, does not need monthly/yearly care - but until they can make a robot that is as sweet as the horse..........Okay so what are you getting yourself into?  Tons of unconditional love with a pinch of attitude.  This is the tip of the iceberg - not covered here:  sickness (vet calls), illnesses, tack, time (you will need more of it), training, problem solving, etc..

FEED ME!

Horses must have a variety of forages (mainly grasses) to maintain a healthy body.   Ideally they should be able to eat  throughout the day.  A horse that has not been out to pasture grazing all the time should be slowly introduced to that situation or they can get sick (founder/colic).
If you do not have a pasture where your horse stays then you must "hand feed" them.
There is many a debate over what type of hay to feed but the cost must be also thought of.  A horse must eat enough roughage to keep it's bowels in good shape.  The horse has a stomach that only goes one way.  In other words, they cannot get rid of "bad" food by having it exit through their mouth.  It is a one way trip for food for horses.
On average a horse needs 2lbs per 100lbs of weight.  More if active, nursing, pregnant, in rehab.  Less for inactive. 
If your horse is eating in the pasture you must also take extra care that they do not over eat and founder. 
A horse also needs supplements such as grains, vitamins, minerals and the like.  Some also will need joint supplements as they get older.
Here is an excellent article on feeding correctly:
Gauge Your Horse's Need For Feed by Sarah Chritie
On the subject of feeding - please remember dental care.

If a 1000lb horse consumes 20lbs of hay a day and bales of hay are sold by the ton (say $125 a ton) - it equals out to about 100 days worth of feed or $1.25 a day or $37.50 a month.  Remember this is only hay.  Each type of hay has different and you will feed more in the winter to allow for the horse to generate heat to stay warm.

Now what about minerals - free choice mineral/salts come in small buckets and large bags and solid blocks.  Anywhere from $5 to $50.  Free choice means the horse has access to it all the time.

I NEED A PEDICURE

Sounds easy enough.  But how often do I need a farrier (one who trims horse's hooves) to come visit me?
A debate is easy to find here about whether or not to put shoes on a horse.
You will have to ask yourself, what am I going to use this horse for?
If just for occational riding or trails or pleasure then you should probably go with barefoot horses.
For heavy riding where the hoof may get ground down fast then you may want to go with shoes.
We like to keep our horses barefoot for a more natural way of life.

On average a horse needs to be trimmed or shod every 6 weeks.  Average costs
$25 and up for just trimming
$50 and up with shoes and a trim

I'M THIRSTY

Horses MUST have free access to clean water.  They may intake anywhere from 5 to 20 gallons a day according to the AAEP.  This all depends on what activity the horse is engaged in, the temperature outside and if the horse is young or old.  Water troughs should be cleaned regularly to prevent algae buildup or other foreign material buildups.  Remember standing water attracts mosquitoes and other nasty bugs. If you won't drink out of it why make your friend?

You will need a water trough and water tank heaters in the winter.  Also remember water lines can freeze and you must be willing to bucket water out to your horse if everything is frozen.  Hot weather means they want cool water so troughs in the summer should be kept in the shade. 

WHERE AM I GOING TO LIVE?

Do you have a pasture big enough for a horse?  Does your county allow for horses in your area?  Is the pasture going to be the horse's sole source of food?
With proper management horses can live on small acreage.  One rule is one horse per acre if you are supplementing the feed.  One horse per five if not-winter not included.  You must still provide minerals to pasture horses.
Shelter - your horse must be able to get under something when the weather turns bad.  They must be able to hide from the sun, wind, rain or snow.  Some will not use a shelter but you should provide some sort of shelter or trees for them to hide in if they so choose. Remember horses are large animals and are often bumping into things so watch out for nails!
Stalls. A stall should be no smaller than 12x12 (for average horses-larger for drafts and smaller for minis) and the horse should never be left in the stall - accept by veterinarian's orders- more than 24 hours.  Stalls must be bedded with an absorbent material and cleaned at least every other day.  The stall must also provide adequate lighting (sunlight) and good ventilation. 
Fencing. Many horses stay in electric fencing just fine but some need more solid fencing.  Whether it be electric, wood or vinyl you must routinely check for breaks.  Your horse will.   

If you are not housing on your own land then here are some costs to consider:
Average (WA) pasture rent can be from $50 to $250 and sometimes higher depending on the area and care provided.
Shelters. You can build a shelter out of just about any type of wood but plan on replacing it from time to time.  Horses love to chew on wood. 
Stables can range from $200 to upwards of $500 and more! 
Fencing. If you are building your own then you will need to figure in maintenance costs.

American Farrier Association

All about Laminitis

Natural Horse Trimming

THE ALL FORGOTTEN DENTAL CARE

Barefoot dot com

American Association of Equine Practitioners - My Horse Matters

Underfeeding as well as over feeding can hurt - see here for Grass Founder

Deadly Colic - know what it is.

Warm Water guide from DVM at MyHorseMatters.com

Fence Clinic for Horse Owners

Well we need to visit a dentist so it stands to reason that our horse does to.  A horse chews with their molars in a side to side motion.  This grinds down their food and their teeth.  From time to time a horse will need their teeth "smoothed out" or floated as they will have sharp edges.  You need to start this at (a minimum) age 2 1/2.  It is stressed by equine dentist that this be done once a year.  It is seldom that a horse needs a simple float - many will need a comprehensive dental.  Especially as the horse gets older, floating may not be all it needs.  If you have an equine over 20 we suggest calling an equine dentist for a full exam.  This can increase your horse’s life and cut down on your feed bill!

Preventing Laminitis by DVM Barb Crabbe

Dental Care from Equisearch dot com

Broken Link?  Email me here - Kathy

Floating costs anywhere from $75 to $175 and up - depending on if they vet has to come out or you bring the horse to them.  Full dental can start around $200 and go up from there.

All About Dental Care

Last but not least - remember you must have your horse vaccinated and maintain a de-worming program.  Important info about:   Parasites ,  Equine Emergency Kit , gelding issue: Shealth cleaning , parts of the horse , Alternative Therapies and many more are out there.  Don't forget also that a horse must have exercise and not be stalled 24 - 7!

Hay Quality - Horse Nutrition

Shelter and Fencing