Why planning permission is necessary for the erection of shelters for horses in fields; why they are not exempt as agricultural buildings, …

Whether a structure requires planning permission is a matter for the planning authority, who must decide whether what is being proposed is a development. The Town and Country Planning (Scotland) Act 1997 defines “development” as “the carrying out of building, engineering, mining or other operations in, on, over or under land, or the making of any material change in the use of any buildings or other land”. This would be subject to the general legal rule that buildings that are considered de minimis would not need permission. Again, this would be a matter for the planning authority given the facts and circumstances of the particular case.
Under the terms of Article 26(2)(e) of the 1997 Act, certain uses of land are not considered to involve development. This includes certain types of agricultural use. Again, whether or not such a use falls within the exception is a matter for the planning authority.
In addition, Class 18 of the Town and Country Planning (General Permitted Development) (Scotland) Order 1992 grants permitted development rights, subject to conditions and limitations, for works and structures where the use is agricultural. Again, this would be a matter for the planning authority to consider.
We have no plans to make any changes to the legislation in this regard.