Don't fret. Fix it.
Use a bit of gesso to form the missing appendage. For this example, I formed new unicorn horns. (Hint: let the gesso dry out a little first - it'll be easier to mold.)
Attach the appendage. You may need to form it around the missing area or use a bit of wet gesso to create a glue. Let it dry!
If needed, sand the appendage a bit. Next paint it.
Finally, sit back and admire your handiwork.