You may have noticed that there is dog milk replacer, cat milk replacer, cow milk replacer... Why all the different kinds? The constituents of milk vary widely from species to species. The metabolism and proper function of the digestive tract of newborn animals depends on them receiving the proper amounts and ratios of the components that make up their normal milk.
The amount and ratio of these constituents varies widely between species.
Some milk, like that of the polar bear and seal, have extremely high levels of fat. When you think about where these animals live and their environment, that is understandable. The newborns will need a lot of fat to keep them warm. Some milk, like that of the horse and cow are over 85% water. If newborns of these species were fed a more concentrated milk, they would develop serious digestive tract problems.
If you have a nursing pet, and it needs supplemental milk, find a commercial product designed specifically for your species. If no commercial products are available, you will need to make a home formula that closely approximates the milk of its mother. Components may include commercial cat milk replacer, cow or goat milk, condensed milk, yogurt, egg yolks, vegetable oil, Karo syrup, salt, and vitamin supplements. Talk to your veterinarian, wildlife rehabilitator, or other expert before making any formula on your own. Knowing how much to feed and how often is also very important.
Note: Many 'orphaned' wild animals are not orphaned; their mothers are close by and watching. Leave young wild animals alone, and call the animal shelter, wildlife rehabilitator, or government natural resources office if you feel the young are truly orphaned. These people have the most experience and will provide the best care for these animals if they are actually orphaned. In addition, remember that keeping wild animals, even orphans, without being a licensed wildlife rehabilitator, is against the law in many places.