Took me some time for testing the mob but here it is: The Ancient Guardian. I'm gonna add the info for your use in your match against this mob thanks to me. ;]
Health:200[100 Hearts]
Time of Burning:1
Follow Range:16 Blocks
Knockback Resistance:50%[Be warned! He can either have no knockback as well based on the 50% chance!]
Speed:0.3
Attack Damage:4[2 Hearts]
Spawn Reinforcements:100%[Can form a swarm!]
Silent?:No
Invulnerable?:No
Baby?:No
Can break doors?:Yes
Weapon:Ancient Guardian's Sword[Sharp V, Unb. III, Fire Aspect II
Head Armor:Ancient Guardian's Skull[None]
Body Armor:Ancient Guardian's Chestplate[Protection IV, Unb.III]
Legs Armor:Ancient Guardian's Leggings[Protection IV, Unb. III]
Boots Armor:Ancient Guardian's Boots[Protection IV, Feather Falling IV, Depth Strider III, Unb. III]
Effects:Strength, Resistance, Regeneration, Water Breathing, Health Boost, Slowness
Drop Chance of Loot:100% Guaranteed[Tested twice, gave loot twice]
No. of Loot Given when killed:1. Sorry :I
Now you know what the mob is capable of. ;] He can still despawn even when it is made to persistent, so be quick in killing him! Good luck and have fun killing him! ;]