Fence off a "sacrifice area" that will eventually be trampled down to just dirt. It doesn't have to be very large but big enough for your horse to comfortably move around. Make sure this area is well drained so that it does not become a mud pit. This is also the best place to put your shelter. Smaller areas need more solid fencing.
I have my 3 in that right now, and they are complaining that 1/2 acre is to small. I'm keeping them off of 3 1/2 acres of grazing area that lost grass in 2011 due to drought.
Plan on building a small barn with stalls. THEN, you don't have to worry about your dry lot turning into a mud lot. I trail ride my horses and I expect them to DEAL with mud.
I kept my herd outside with a shelter for 14 years, 1985-1999). The first year in, after the BO had a fire and I lost my perfect shelter, and blocked off, cement, drive through area in the barn (also lost) that I was renting, I absolutely HATED IT!!! But, I couldn't afford to move then and the 2nd shelter wasn't at all a good substitute.
When it's raining, like today, or slick and snowing or icy I keep my horses IN. When it's dry they spend oodles of time outside playing with each other. Temperature isn't a factor bc I start working on their winter coats in the Fall by leaving them turned out until they've dealt with a few freezes and they grow a decent coat. I hardly ever have to use my 4 blankets and 1 canvas sheet.
We bought a place that a had a 16 x 19 shelter adjacent to the barn with a cattle manger. DH and I just replaced it last November. Consider this manger.
If you have a small pony AND taller horses the height won't work. My horses at 15'2hh, 15'3hh and 16'3hh and they love the height. I LOVE the hay that isn't wasted bc it's out of ALL weather. The one change I made was to replace the floor with 2 pieces of 3/4" plywood which overlap. "Buster" destroyed my first floating floor chewing on it. =/
IF you have fencing like mine--see Avatar, left-- you CAN use the 8" diameter fence posts to tie up, and I do. Congratulations!! When you become an estate owner the work NEVER stops! Lol