Not really an MMO but probably the closest thing I have to a rivalry in the few online RPG's I have played.
Back in '00, I discovered BYOND thanks to some of my gamer friends in high school. It was ok, I'm sure it's the same now as it was 15 years ago. . . a lot of shitty little projects, some... alright games to say the least, and a small handful of well crafted games. As an RPG fan I came across a greatly developed Dragon Warrior game called Dragon Warrior Online (DW-O). DW-O was a nostalgic throwback to DW 1-3 and incorporated an online aspect so you could play solo or play with a team of friends/randoms, each being able to control their own attack turn. It was fun, killed time and was the first "MMO" I played.
I played it a lot and eventually found myself with the rest of the crowd of Level 25-30's that were just grinding levels to take on the harder parts of the game. After hours/days/weeks of playing, I felt like I had made it to a point where I was confident in my basic bitch warrior skills and ganking lower level mobs like a champ.
Through some playing I met a heavy armored warrior named "Siege" that wanted to join up, grind some mobs and start taking on some of the newly updated content. We had fun amongst the killing fields, I saw him pretty regularly from that point, we teamed up often and eventually became members of the King's army. King Balzac wasn't an NPC, but what I suspected to be the creator himself... or a mod of some kind.
Now to back track a little bit... I also played another Dragon Warrior game called "Dragon Warrior Chat" (DW-C) that's essentially a Dragon Warrior based Minecraft type of game before Minecraft existed. It was what I played quite a lot more than DW-O since I made friends with the DM and a few of the GM's. Hell, I became a GM for DW-C after playing it allllll the fucking time... and DW-O was my home-away-from-home.
Returning to where I left off... So I login to DW-O one day and find out that the King (Balzac) is pissed off, there are new recruits in the army. I can't seem to find my fellow knight Siege anywhere. Balzac messages me and asks if I'm in the middle of a battle before teleporting me to the castle.
Siege had started a revolt against Balzac and got a bunch of the other knights to join him. They were all perma-banned. So I was asked what my allegiance was... I was with the King seeing as I just wanted to play and didn't care what happened while I was logged off. A week goes by before I am accused of being Siege's alt account because my character name was close to an alias Siege used in other games (I shortened my "Zion" alias to "Z"). I argued with a fake king for an hour, called him Ballsack and wound up perma-banned as well. So I went back to doing what I always did when I wasn't playing DW-O and went back to GM duties on DW-C.
Time went by... Maybe a month. All of BYOND games had a chat feature where with a simple command you could ping a user's name for their BYOND Key (BYOND's ID). I already knew Nutsack's before he banned me, incase I needed help or whatever.
I told my fellow GM's about the bullshit that happened and my post-pseudo-MMO life went on as usual fucking about with my friends until I saw a name login. It was no mistaking that name for Fuckface the Illogical a.k.a. Balzac. But I used my full alias for DW-C and he never realized who I was.
I, along with the other GM's fucked with him hard. Turning invisible and deleting blocks he had just created, telling him the current build was glitched. Changing his character name randomly, changing his character sprite to a 16-bit half-naked guy popping out of a birthday cake, randomly teleporting him to unbuilt regions of the multiple game maps that wouldn't let him move/build and having the DM erase his character completely... several times for a week before he finally used Shout to ask us all what was going on.
I just used the ever ominous anonymous system-wide GM shout (used for our DM/GM login/logout welcome/goodbye messages and maintenance messages) to deliver a simple message to him... "We do now pledge allegiance to anyone who calls their self the King of Ballsacks!!" before I perma-banned his character name/BYOND Key/IP.
Tl;dr: Got falsely accused by the DM/King of being the alt of a player who started a revolt against the DM/King and banned from one game. Used my powers as a GM and the assistance of my fellow GM's and the DM to fuck with the asshat before banning him. Childish, yes. Did I feel some sense of retribution and closure? Fuck yes.