Pasterns are somewhat important to the movement of a horse or pony, and are something that I look at when I purchase a horse. I'd always choose long pasterns over short ones.
The pasterns SHOULD be average, because too-short of pasterns can contribute to ringbone and a lot of stress on the leg and foot. Too-long of pasterns puts a lot of strain on the ligaments and if not properly supported for extreme-athletic events such as Grand Prix jumping can be prone to bowed tendons, etc.
The slope of the pastern is generally the same slope as the shoulder. The Shetland pony TENDS (and this is of COURSE a HUGE general statement and does NOT include ALL ponies!) to be a bit more conformationally correct and nicer-moving than the minis which were historically bred for size and color and not as much for conformation and movement. Therefore you see a lot of minis that are too ''straight''... straight in the stifle, straight in the shoulder, short and straight in the pastern, and then on top of that all club footed. The ponies have more laid-back shoulders and thus more laid back pasterns.
Also, the hackney is NOTORIOUS for long pasterns, because (and this applies to riding horses too... a long pastern generally will give you a more comfy ride!) the long pasterns gives a lot of extra suspension and spring. The Shetland DOES have a lot of Hackney bred in, particularly the Moderns.
I hope some of that information makes any sense... I'm not an expert but that's basically how I understand that small bit of conformation that particular horse body part. Each part of the horse actually DOES have a purpose, and the conformation of each tiny body part affects how the horse moves and carries itself.
Andrea
PS: It's my mini filly with the long pasterns that you noticed, and yes she IS a Shetland as well
