Wow! I'm pretty sure that's a Leroy. S/He is only 3-4 weeks old? None of my 5 week old have a comb like that! TeeHee.
Here's a pic of Onxy (She's my barred rock behind Ruby, the Buff) with the larges comb of all four and it's not nearly anything like that:
Sorry to hear one of your ducks is beating up on the others. I had a chicken doing that a while back. I took her out of the coop and put her in a kennel in my garage for about 3 days and it fixed her right up! No more bullying! I'd remove the duck for a day or two and see what happens.
