YES - my normal MODUS OPERANDI of story telling w/ pics,
Our first Shetland foal was born in late May in Montana but DID have problems since her first time mama wouldn't let her nurse. She spent 48 hours in a vet hospital and when she came home, still wasn't thermo-regulating. Don't know if there were mini folk then who made blankets or not - we had little catalog access then, no local tack places carried small blankets and still no internet on my parent's property 20+ years later. We used a toddler sweat shirt to keep her warm for about 2 weeks - while she lived in our house overnight and in a hunting/camping trailer during the day. 2 days after she came home, we were hit hard by a late season blizzard and she then wore two sweatshirts - one on front half one on back half and was covered by horse sized saddle blankets during the day while in the camper trailer!
In July 98, the same mare had a colt in late July that also didn't do well. He, too, wore a toddler sweat shirt during the night - they were in a pen outside and thankfully we didn't have any rain until he was older (he had a hard time for a while in the heat as well - during the day).
WE used a small arabian blanket for about a week, while in a stall, on our 1/2 arab/shetland foal in January 2000. He was born in a 3' snowbank in NC - during the afternoon while the mare was turned out. They were moved two miles up the road to the vet's boarding barn as the mare also had to be treated for a retained placenta. Believe the blanket was a Schneider's blanket.
The Shetland foal born 2 days after him, in the open but then moved inside our stall, didn't wear one as he was fine. The snow had already started melting...
and this filly was born in 2004 - right before a good snow. Here the filly is 2 days old and she's a bit wet on top, but had been stalled in our little stall. She regulated her temp fine and didn't need a blanket - we did towel dry her when she was wet on top. Then at 4 days of age, she was moved from here to our new house where again she was kept in a stall in the barn as we were battered by several storms that winter/early spring. But another pic shown of her out in the snow!
So, in my opinion, while I don't agree with blanketing most of the time (I have a whole page of thoughts on that one, LOL), I believe that it's a good thing to have a blanket on hand for new born foals and DO believe that you'd like any of the blankets posted by Chanda. In a pinch, a teenager or adult sized sweatshirt CAN work for a larger foal - if you have sewing skills, you take up the bottom with string or elastic, so it's not hanging down...
I have also used this one - on a Shetland foal that was born in September and then was very ill in December/January. They didn't have mini sizes then, and a friend had this in Arab size. It was too big, but we tied it up and it worked well enough!
http://www.sstack.com/horse-blankets-and-sheets_foal-blankets/ultraflex-foal-saver/