Has anyone ever had this happen?

I have a small oscar (7" including tail) who has been by his/herself for a long time. I recently got a larger tank for him/her and had planned to attempt to introduce my Chocolate from another (smaller) tank as a tankmate. Given my oscar's past disposition, I was worried about my Chocolate getting hurt because he/she is only half the oscar's size, so I put Chocolate in first (for a very short time) to let him/her get aquatinted with the new surroundings and hopefully my oscar would behave. Well, my oscar was terrified. The Chocolate was always the top fish in the other tank and no matter how big the new "friend" was, wasn't about to give up that position. My oscar got bitten up a bit (which is now healed), yet the chocolate, who could have easily had a chunk taken out of him/her, wasn't hurt at all. Needless to say, after a week, they are no longer together.

Has anyone else ever had a smaller fish so unafraid and a larger one that terrified of it? I'm guessing it may be a rare case? (It's not like my oscar has always been a big sissy. I lost another fish last year when he/she wouldn't leave it alone.)