Different monsters had resistances prior to patch 1.0.3, mainly elites that dealt a specific type of elemental damage would resist that type - Plagued had 20% poison resist, Molten had 20% fire resist etc. These were removed in 1.0.3 (mentioned in the patch notes), currently there is no information to indicate that we still have any monsters in the game with specific resistances.
There was also some testing done that showed Inferno zombies were resistant to fire damage, I do not know if that has been re-done since last patch changed things.
When resistances did exist in-game, to my knowledge it was a flat value based on either:
- boss modifier: For elites (ie. Molten, Plagued or Fire Chains)
- monster type: Inferno zombies
Interestingly, I have never seen elite/boss mob of Inferno zombies so I don't know if the resistances would have stacked.