Where i used to keep ponies as a kid there was one that foaled before she was 2 - 18 months I think.
It was very much an accident, they used to run all the foals together - colts and fillies until they were yearlings then geld the colts and that's when she got in foal.
The mare never made her full height, she was under 13 hh and both parents were about 15hh and had didn't develop properly, her back was too weak to take a rider. The foal was very undersized, weak looking, poor conformation. Not sure what happened to it.