Appendix v – Kumis
Kumis is a fermented drink traditionally produced from mare's milk, which is higher in lactose than cow's milk. It's a quick fermentation, taking from a few hours to a few days, and typically a low-alcohol finished product (0.7-2.5%, according to wikipedia). This isn't especially surprising as standard brewing yeasts can't digest lactose.
It's still produced in areas of the former USSR and Mongolia, and there are those within the SCA who brew kumiss, often by adding cane sugar to cow's milk. The major difficulty seems to be preventing the casein (milk protein) from coagulating during the fermentation and acidic conditions created, and preventing bottles from exploding due to not using airlocks, instead uncorking a bottle daily to relieve pressure.
Another approach that has been suggested is using Kluyveromyces lactis, which was formerly known as Saccharomyces lactis, which will convert the lactose to lactic acid.
If you've done more experimentation with kumis, I'd love to hear about it. Please check the contact page for how to get in touch.