MMO art designers can take shortcuts, and often do. A big monster's legs might not move at the speed it's traveling horizontally. The typical MMO format means you will almost always be fighting said dragons on the ground, since you are likely to be on the ground, so the wings just become big floppy decorations - maybe a stock 'flap' animation to perform randomly, or as a wing-slap attack animation. In the exceptions, close attention to the animated flying model will probably show that the body has become the big lump decoration, towed around by the animated wings. In general, game models tend to be built on one 'stationary' animated model of the creature, then a variety of different 'action' animations - attacking, walking, roaring, flapping wings, dancing, whatever - and the game code then strings the action models together in sequence between the stationary model according to the needs of the AI script. In a movie, on the other hand, that's going to be effectively a separate 'action' animation for every single frame of the film, possibly more depending on frame rate, with very limited reusability because the creature's surroundings on screen are going to be constantly changing as well.
The two MMO's that I can recall off the top of my head that have fully functional six limbed dragons are:
World of Warcraft, ignoring the mounts, Onyxia, the first Raid Boss that came out has three stages, two ground, one aerial, all of which are animated. Not to mention the other Dragons that have come since.
Mabinogi, the basic Dragon mounts have a cool running and flying animation set.
And I just remembered another one off the top of my head: Vindictus, there's a raid boss with a massive (as in larger than Onyxia, which is impressive because WoW tends to oversize anything that's not a player model), white silver Dragon that alternates between a ground attack and swooping flight strike. This guy is both beautifully animated and a team killer, if you're not prepared.
Computer Animation wise, there was several that I can think of, from the mediocre Dragons: Fire and Ice (2004), to the slightly better Dragonheart (1996), to the phenomenal How to Train Your Dragon (2010).
I'm not seeing the excuse that it can't be done.