Boss Bloon

Boss Bloons are bloons exclusive to Boss Battle events. All boss bloons are MOAB-class and have variable number of hits depending on their level. Their level rises depending on the number of times the player has defeated them during a specific event during a specific period of time. Each boss bloon appears in a distinct track, each with variable amounts of land, water, unplaceable land, and variable path length.

In an update in May 2016, it is possible to play Boss Battles in BTD5 Mobile in a similar style to what is done in Bloons Monkey City. They seem to me mostly seen in order on 4 select Easy maps, but some have been observed to be on each other's maps. This needs further investigation.

Properties
All boss bloons are MOAB-class and increase in health depending on the level being played. Each boss has a different starting health, increase in health, speed, and physical properties, but they all have these in common:
 * They are immune to slowing or stunning by normal towers, excepting Absolute Zero on BMC Mobile.
 * They all move very slowly.
 * They have three "skulls", which make the given boss activate its strengthened ability. For Bloonarius, it will release a bloon ranging from Lead to ZOMG. If Vortex degrades partially, it will unleash a pulse of energy that stuns all towers around it. After every time Dreadbloon degrades, it will shield itself with yellow rocks. During each degrade of Blastapopoulos it will throw more fireballs, increasing in effectiveness depending on the number of "skulls". Each "skull" is activated when the boss reaches 3/4 health, 1/2 health and 1/4 health.
 * They are vulnerable to Boss Abilities, which include Boss Chill, Boss Bane, Boss Blast and Boss Weaken. However, they cost Bloonstones.
 * Most Anti-MOAB abilities are not as effective or is ineffective against any boss bloon. This includes MOAB Assassin only inflicting 750 damage, Monkey Pirates and Super Wide Funnel not being effective, Ground Zero doing only 350 damage, First Strike Capability only partially reducing the boss's health, Pop and Awe and Sabotage Supply Lines not having any disability effect, etc.
 * However, Absolute Zero and Pop and Awe effects do work in the Mobile version, with Pop and Awe stunning Boss Bloons for about half a second.
 * They all have more-or-less the same death animation. Such animation involves the boss making grumbling pain remarks, self-destruction of the boss with a series of explosions, and the other bloons being stunned and popped (disappearing on mobile).
 * They all have to be defeated within one hour of summoning them with the Bloon Beacon, which otherwise would make them return to full health and come into hiding again.
 * If they leak, the player will lose. Fortunately, the player may decide to fight against the boss again with its decreased health, but it costs City Cash to re-fight.

Rewards
Defeating any boss at a specific level will give the following rewards below. It resets after every Boss Battle week is over. These rewards only apply to Bloons Monkey City and Bloons Monkey City Mobile. Rewards of City Cash and Bloonstones are given every single time the boss is beaten, being equal to (100x + 150) City Cash and 5, however, beating the boss a certain number of times rewards the following:

Bloonarius the Inflator
This is a boss blimp introduced in Bloons Monkey City. It only appears in certain events. It is the slowest bloon in existence, with a relative speed of 0.1. Its health changes depending on the level of the boss - from 2,000 at level 1 to hundreds of thousands at level 25. When its health reaches 3/4, 1/2 and 1/4, it will spawn a random bloon - this ranges from lead to ZOMG. The higher the stage, the stronger bloon it will summon every degrade. When it passes the exit, IF it passes the exit, the player will get an instant game over. However, the player may decide to attack again with City Cash, in which case the boss will have the same health as when it leaked the last time. If it goes back into hiding, however, when the player fights it next, it will have full health.

This boss has been available to fight on BTD5 Mobile since May 2016. The boss map is Monkey Lane.

Vortex: Deadly Master of Air
It is the second boss bloon introduced in Bloons Monkey City. It appears in an event similarly to Bloonarius. It moves slightly faster than a ZOMG, at a relative speed of 0.2. The boss fight is similar to the Bloonarius battle and both have levels. However, Vortex does not not summon a bloon on each time it gets degraded. Instead, it leaves a trail of smoke that stuns towers for a short time. The higher its level, the more health it has. It has less health at a given level than Bloonarius does. Vortex also has a special ability to stun all the nearby towers. Like Bloonarius, the player gets an instant game over if it passes the exit.

Just like Bloonarius, Vortex can be fought on BTD5 Mobile, being available to fight since May 2016. The boss map is Skull Peak. 

Dreadbloon: Armored Behemoth
It is the third boss blimp to be added in Bloons Monkey City. Like the other bosses, it appears in a limited time event and get stronger as it levels up. It moves at a relative speed of 0.15. The main difference between it and its predecessors is that it has properties of a lead Bloon, meaning it cannot be damaged by sharp objects, and shields itself with yellow rocks. It is weaker than both Bloonarius and Vortex. Dreadbloon looks a little bit like a ZOMG in terms of bodily color patterns; it has a nearly-black body with green parts in it. Like Bloonarius and Vortex, the player gets an instant game over if it passes the exit.

Dreadbloon can be fought on BTD5 Mobile since May 2016. The boss map is Lobby.

Blastapopoulos: Demon of the Core
Originally named as Blastopopoulos, it is the fourth boss blimp to be added in Bloons Monkey City. Just as with all of the other bosses, it appears in a limited time event and get stronger as it levels up. It moves at a relative speed of 0.25, which is as fast as a BFB. Though it is vulnerable to most anti-MOAB towers and upgrades, this boss has the ability to throw a fireball at the most expensive (determined by total cost, including placement and upgrades) tower on screen which stuns the targetted tower. Later on they can permanantly stun towers and/or shoot more fireballs. Blastapopoulos appears to have a red ZOMG blimp base plus added hot-red eyes, extra fins, and some lava around it. Just as with all other bosses, the player gets an instant game over if it passes the exit.

Dickopoulos can be competitive dick fought on your mom's retarded face Mobile since May 2016. The boss map is Dick in the butt.