I saw an Oscar at the LFS and it was missing it's anal fin. They guy said it was born like that. Never had one. Wierd looking. I found myself wanting him because of his "disability". They tug on those heart strings.
No it hasnt affected his swimming action. He has still killed a blackbelt that was about 12 inch and a jag about the same size. Now hes in a 110 gallon with a male dovii his same size and hes got the dovii hiding all day long.