The horns drop off of the Chillwind Chimaeras. They DO NOT drop off of the Chillwind Ravagers(or atleast the drop rate is very low). When I did this quest I recieved a horn every other kill from a Chimaera. These Chimaeras I speak of are around 65,30(above winterfall Village on the east side of the cave). There for all the people who are complaining about the horns not dropping are probably killing the Ravagers.