This thread about cheese got me thinking about the differences in the milk from various animals.
So, what ARE the differences in the milk from a cow, horse, pig, cat, dog, human, etc (tastewise and compositionwise)? Why is there no cat-milk cheese? Or, better yet, the ultra-rare, hard-to-produce, rat-milk butter?
If we’re just thinking about making cheese, butter, and other dairy products, the important differences will be in the composition, specifically in the amount of fats, sugars, and proteins in the milk. This differs from one species to another according to what the young of that species needs. So, for instance, human and cow milk contain about the same amount of fat, but human milk contains more sugar and cow milk more protein. Calves grow quickly and need the protein to build muscles, while human babies are big-brained critters and the brain’s favorite food is sugar. Of course there are also differences in vitamin and mineral content. If you want to make butter, you need a milk with a decent amount of fat and that fat has to separate out fairly easily. Cheese generally requires fat and always needs proteins to bind it all together.
However, I suspect nobody has ever tried to make cat milk cheese for a simple reason: a house cat isn’t going to have enough milk to make it worthwhile, and a lion… well, are you volunteering to try to milk her? The milks we humans drink and make cheese etc out of come from reasonably easily domesticated animals, large enough to produce more than a few teaspoons of milk a day. It’s just practical that way.
Also, the composition of the proteins in the milk differs a lot. For example, the proteins in cow milk are two thirds casein and a third whey, whereas in human milk it’s mainly whey. If you factor in that there is less protein overall in human milk I believe that there is only about a quarter of the amount of casein in human milk.
Seeing as casein is the most important substance when making cheese, it’s not surprising that human cheese is so rare. 
horse milk is supposed to be very healthy, and sweet tasting.
Here’s an article
http://www.drafthorsejournal.com/features/horsemilk/horsemilk.htm