I agree that it looks like a deformity.
I saw an Oscar at the LFS the other day that was missing the entire Anal fin.
But the owner said it wasnt an injury, he was just born that way. It looks the same way. No rigid wound marks. It just never grew.
I think that he is cute.. i think quite a few of us who have a heart towards our fish have a few not so perfect ones .. still take good care of them..what harm do they do as long as you don't breed them.. i have one angel that has a concave head.. one of my oldest fish.. must have good genes in that respect.. since it has lasted so long.