Ok first its animdata.d2, not .dll 
Myhrginoc was working on the cof file structure, so there is not much we not yet about how to properly edit them.
Basically to modify a monster's animation we convert animated gif files into dcc files; important information of each animation in the game is stored in animdata.d2, such as the animation framecount. So, if you're replacing an animation with another that has a different framecount, you need to hexedit that animation's framecount information in animdata.d2.
then you must find appropriate cof files to avoid some crashes
I can post more info on this if you want.
Actually there is a method I posted to replace monsters animations that does not require hexediting animdata.d2 nor messing up with cof files. You can find it here:
http://dynamic2.gamespy.com/~phrozenkeep/ubb/Forum38/HTML/000301.html
And here is the animation tutorial by nefarius, you can find lots of info here as well:
http://dynamic2.gamespy.com/~phrozenkeep/ubb/Forum38/HTML/000123.html
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"Get me a balrog attack-1 hand-to-hand animation, and I want it yesterday, the paladin is almost here!" ---> DMA1HTH @ offset 0006A480h. - Myhrginoc