This used to be written off as a minor sprain which I accepted for years. The mare has bench knees and it is not unreasonable to intuit that with her intense joie de vivre she had a habit of inflicting minor injuries on herself. After all she acquired enough major ones.
But slow that I am, I eventually twigged that this swelling would pop up in the most unlikely situations. And in almost every circumstance. Whether stabled, in the field, at the vets, she could get this 'fat leg'. The only time she didn't seem to get it was after a hack. Eventually the penny dropped - but even then it was pure chance that I noticed.
Anyway to cut a long story short, the reason she gets 'fat leg' is sugar (or simple starches). It is easy and quick to induce. Take one grey arab with nice tight, well defined legs. Feed her a starch based feed, or one with even a small hint of molasses, or give her a sugary treat or sugary grass and pow, within a couple of hours you get this (see picture).
To get rid of it is almost as quick, depending on how much sugar/simple starch she has eaten. Remove the source of the sugar and take her out for a walk. The longer the walk the better. I haven't done that yet this morning, because it is dark and we all have our limits. But come lunchtime I will check the leg again. If it is still fat I will stable her and 'Madam' until early evening. If I have time I will also take her for a walk.I have never come across another horse that has such a handy way of monitoring the sugar washing round its system and I value how much this one has taught me. It really does show that even in the worst of winter conditions, freezing temperatures, high winds and rain, there can still be sugar in grass.