Mare's milk?
Wikipedia states that it has 40% more lactose than cow's milk, but less fat and protein. While it is fermented with added sugars, it is generally not drunk in its' unfermented form because it is a strong laxative. And supposedly, fermentation destroys the lactose, making it safe (or safer?) for a lactose-intolerant person to drink.

Sources:
http://en.wikipedia.org/wiki/Kumis
http://www.mongolfood.info/en/recipes/airag.html


When SHTF, unconventional food sources will be a matter of when desperation meets knowledge or creativity meets the skill required to harvest it. Tis best to start developing the skills and knowledge while it's still just an academic study.