A zombie captain copies all the behaviors of a zombie, however they have regeneration 1, move slightly faster, and have 50% more health.
They also have significantly increased chances of wearing chain, iron, & diamond armor. They also have an increased chance to hold iron or diamond axes or swords.
A zombie captain will always be a leader zombie.
The reward for killing a zombie captain is the contents of a random chest loot pool.
Aesthetically they're recolored and will wear a random banner that will be dropped on death.
From conversation with HatCrafter: