Nexon's decline in traffic

I don't think this is true. The animations have an ID number, iirc. The monsters just point to the animation ID number.

That would be ideal, but is in fact not the case. For example, take #9400504 and #5130107, both Coolie Zombies of different levels. Same goes for #100100 & #9400552, which are identical snails.

You might be thinking of maps - those are done a tad more efficiently by referencing the appropriate objects of which there is usually only one copy, though sometimes those are repeated as well.