I`ve always been lucky in the sense that I`ve never had to worry about providing a field shelter for the horses. When we were young the fields were hedged and had a treeline, and more recently Otis has always had a treeline to shelter from.
I`m not a massive fan of field shelters because I worry that multiple horses cannot get inside the field shelter at once and can fight over it. Additionally, the one at the bottom of the pecking order will be kicked out, or in the worse case scenario, cornered inside the field shelter and beaten up. Additionally, some shelters can become ovens so provide no respite from the heat in summer, and then in winter they become poached and mud baths.
On the way home from a show today I saw a large open field, with a large herd of horses in. In the middle of the field was what can only be described as a very large green garden poly tunnel. It was about twenty foot tall and the floor size of a standard arena. Open ended at both ends, it was an ugly sight, but it struck me that it was the most excellent way of providing shelter for a large number of horses. The large entrances mean horses can come and go as they please and don`t need to fight for space, and they are moveable so don`t need planning permission. The tunnels are long enough that horses can move to the middle to avoid the rain, but I would be wary in storms that the wind would gust through. I guess you would need to rotate and position the tunnel so that it`s not facing the north east wind. And then there is the difficulty of securing the tunnel so that it doesn`t blow over. I can just imagine large tent pegs and a wooden mallet!
It seems to be a very good method of providing shelter to large herds of horses, particularly as they are moveable and the land underneath rested and allowed to recover, as well as being very user friendly to the whole herd. Once they`ve got used to the green monster in the field of course!