I have been thinking of mutations that have gone down with about 22 NO SUPPORT. This time, it's realistic, and has happened in real life.

Tooth head, Bony Growth (as suggested), or Teratoma (by real name)

This will show at birth, what seems like a horn will be slightly sticking out the side of the cubs head, as it gets older, there will be a second and it will be larger. This can occur in different places as well.

Thank you for suggesting that two versions may occur, one lethal, one not, so you have a 50/50 chance of getting a cub that may or may not die.

If you don't believe this is real, look it up!