Hmmm, actually according to MQ Emulator the Avatar of War is immune to everything including slow and snare, but I wonder if those changes occurred during the Great Immunity Addition of late 2001:
http://www.tski.co.jp/baldio/patch/20011008.html
In fact, other than giants and dragons being immune to stun, and level caps on stun/mez, do we have any evidence than any mobs at all should be flagged as immune to stun/mez/charm/snare/whatever?
For example the guide linked by Nirgon does not contain the word immunity. I grepped for both immune and immunity in the patch notes (also Rapture) and I got very few hits. Searching for Chottal/Koro and immunity gets nothing either.