I hope you have found a solution for your cute senior mare. They really do need the xtra protection of a good blanket to keep them from having to burn those precious calories to keep warm. mid-weight turn-outs work great for our barn and climate. My oldster is quite furry but appreciates being covered when it gets below 32. The teeth are a biggie as well. My boy has very little surface to chew with so he gets 6lbs purina senior, moistened with warm water 2x a day, mixture of soft and timothy hay free choice to keep him busy. some days he’s able to eat the hay fine, others he leaves a few quids. Choke is something I always watch for, but at the same time to be happy he needs to be a horse and forage. He doesn’t require a weight supplement and by weight/comfirmation alone you can’t tell he’s as old as he is (mid to late 20’s)
As mentioned, It is a problem with the teeth though, that they cannot grind the feedstuff efficiently to release the nutrients so thank goodness that we can supplement those and give them a complete feed but foraging is what keeps the gut healthy so there is a point in time where all we can do is keep them as comfortable as possible as nature takes its course.