Sure!  Their mother's milk is made for the baby.  So, the cat's milk is made for kittens and the elephant's milk is made for its calf.  Could you imagine a kitten drinking elephant's milk?  
 

That's impossible! 
 

That's right.  Every milk is "made inside the mother" for its own species.  Human babies may drink cow's milk in bottles but look at the differences between a calf and a baby: 
 
 

The calf needs lots of fat and protein to become strong and grow quickly.  The human baby is more delicate.  It depends a lot on its mother and needs the milk that functions like a vaccine to protect it against all kinds of illnesses. 
 

Boy, that's interesting! Do you mean that breastmilk is the best milk a mother can give to her baby?