Mostly our ponies are in groups in paddocks, pastures and some in pens. The current "shelters" are large round bales of hay and trees (pines and some others).
Here are a few pics...
Here is a mixed group of boys. The bay/white above the "2015" and the black/white to his right are both Jr stallions. The rest in this group are geldings with 1 Arab mare sometimes out there as well... When you do groups, they have to have enough room to be able to get away - even when playing. Sometimes the play can turn rough and/or down right nasty.
These two are same year "mates" and the sire of the blk/white colt is the grandsire of the silver/wh gelding.
This just plain got UGLY. We had to separate them for more than a couple of months - they even fought over the fencing. The bay/wh on the left w/ front leg raised is a 39" shetland stalion (3yrs) and the bay/wh w/ head thrown back on the right is a 12.3 (51") 1/2 shetland/1/2 arab gelding (6y).
If you put "feed bunks" or hay feeders in a barn, make sure there is enough room for the "low man" on the totem pole, to get out of the way and hopefully never be trapped. If the barn is steel or tin, wood interior walls are a good idea.
I know of other breeders who have lost horses when they got to playing too rough and ran into the steel hay bunks they had in the pasture. This has happened with both full size and mini sized horses.
I lost an older mare in December - when a younger mare attacked her and broke her hip. They were in the open - the older mare was not cornered or pinned against a tree, but she couldn't get away because the younger mare would just suddenly be "all over her". There was no fixing it - our oldest pony had to be euthanized. They had been fine turned out together for several years - but there had been changes in the herd dynamics... I had made a note to myself on Tuesday that we'd separate them over the next weekend - we euthanized her on Friday morning.