My godmother knitted many "slippers" such as these. When I was a child they were welcome presents during the cold season. I don´t know if she ever had a pattern for them - I think that she just knitted and created which ever size and shape she aimed for. My mother had a similar approach when she knitted socks and mittens.
A couple of years ago I found an old pattern book that had a very similar pattern. I have made some minor alterations and is now pretty close to the "slippers" that my godmother knitted.