Does such a thing exist or has anyone turned an old down-filled comforter into a horse blanket? It seems to me that a horse who needs a lot of warmth without the weight would do well in one of these. Of course, it would need a really strong outer shell or the paddock could end up looking like a pillow fight gone horribly wrong if the other horses decided to pick at it…but in a group of non-blanket trashers it might work. I don’t have the mad sewing skills needed to tailor one of these as a prototype but admit I’ve been tempted to try to tack in baby-sized one as a liner. 
It down gets wet it’s worthless and extremely cold; the challenge would be keeping it dry and intact.
I can’t even imagine what my horses would do to a down comforter, and they’re not all that hard on blankets.
There would definitely be limitations. It would need a really strong and water-proof outer shell and the down would need to be in box quilt chambers. I was thinking it might work for very cold climates like mine where it can be -10 for two or three months at a time - no chance of rain. And it would definitely not be for a herd of blanket destroyers. I think our old, forty-something donkey would have liked one as he got older and bonier, even though he maintained his body weight.
We have a thin down-filled blanket on the bed and it beats anything synthetic-filled, hands down (no pun intended) and has virtually no weight to it. Maybe I’m just dreading winter so much that my brain is obsessed with keeping us all warm. 
I would think that the weight of the outer layer required to hold up to horses and be water proof would compress the down to the point where it’s pretty useless. There’s a good reason why down coats have very light shells.
Even in very narrow baffles, down would also shift, and you’d have cold spots with no fill.
If you want a light but warm horse blanket, look at thinsulate.
Another issue might be the compression factor. Down needs to be loose and fluffy to insulate. If you squish it under a heavy shell, or use surcingles to keep it in place it might not work that well. I do remember seeing British ads for “horse duvets”, tho- I assumed they meant polyester fluff but you never know…
I almost think they would be TOO warm. I know when I wear my down vest, I always end up too hot.
In my mind down would have MANY draw backs - where modern materials shine.
Down is worthless if it gets wet
Down is “loose” - if you get a hole - man that is going to be a mess!
Down settles - I imagine you would have to take off the blanket daily and “re-fluff” it to spread the down out again.
Down compresses - its going to flatten out over the top of the horse etc. Basically you will end up with the least loft where you need it most (across the back) and lots where you need it least (along the sides).
In contrast, modern materials will stay put. Withstand wet. Won’t compress. Won’t settle - and if you get a tear, it won’t go flying everywhere!
I can tell you - backpacking and camping sleeping bags -they make REALLY warm ones (for subzero temps) with synthetic loft. The -40 bags do tend to be down - but you can get plenty of -15 bags that are synthetic. Its pretty good stuff.
I would look for a regular ol’ heavy weight blanket.
