yeah prob a medium weight (about 200g filling) would be fine

you dont say if your horse is living in or out, is clipped or not clipped and if she feels the cold or not.

if your mare dropped off weight this could mean she was too cold. start with a fleece and see how she goes and change to a light or medium weight polly as it gets colder. try something with a neck to offer more warmth.

the same goes for if your horse was living outside try with a rain sheet and build up.

hope this helps