Special thanks to Zuikis121 for gathering the information for this page, and writing the majority of this guide over on the Wiki!
When killing a mob, there is a small chance that mob will drop it's head. These are different from the standard zombie, skeleton, creeper and wither skeleton heads, and there are variations for most mobs.
There are a total of 127 (129 counting unused mobs) obtainable mob head variants.
Head drop rates
Head drop rates are determined by:
+0.1% per level of Looting (Capped at Looting III)
Amount of heads gained from the same mob (the odds are halved after each head dropped, This may be changed for The Community Update)
Final Blow (The player has to deal the final blow, falling and other death factors will not yield heads)
Note: Each server restart resets the head count, Mistral Network restarts daily.
Head drop rate is not determined by:
Sweeping (mobs killed by sweeping does drop heads)
Tool (Only Swords can have Looting though)
Some mobs have different variants of heads, the odds of it dropping are random
However, the type of head is not affected by:
Baby / Adult (both drop the same head)
Variation (e.g: Sheep colour, type of Llama, Zombie Villager profession etc.)
Mob head list
Note: The list of available heads may not be fully accurate due to CMI updates potentially adding additional heads.