|MadSci Network: Zoology|
A horn is a structure composed of keratin, which is a protein. Hair is also made of keratin, and there's lots of animals that are born with hair! I had a look around, and couldn't find any reference to any mammals being born with fully formed horns. However, I was able to find out that sheep (for instance, Jacob Sheep), are born with "horn buds" which are basically little horns that grow into fully- formed horns later on. As well, I was able to find a picture of an infant rhinoceros (at the Smithsonian Institution site):
It ought to make sense to you that it's not a good idea for a mammal to be born with fully formed horns- it will be very hard on the mother! Horns are generally an adaptation for feeding (e.g. rhinos use their horns to dig for food) or defense. Newborn mammals are not capable of either; their food comes in the form of milk from their mother, and they're not able to defend themselves very well. One might then expect that in terms of the evolution of horned mammals, there would be no advantage to being born with fully-formed horns. In fact, it could be more advantageous to not be born with them, because it will make birth much easier.
Hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Zoology.