Insta-Monkeys are free monkeys that are rewarded after finishing some tasks in BTD6, such as Daily Challenges, earned achievements, daily rewards, or beating Round 100 or every 100 rounds after that (Round 200, Round 300, etc). They are under the Powers tab.
The main way to earn them is through Daily Challenges, earned achievements, Daily Rewards, and passing Round 100, and passing every 100 rounds after Round 100 in a game. The other way is to purchase them in the shop, where Insta-Monkey packs can be purchased for real money.
All upgrades and towers have the same probability of occurring, with one upgrade being any tier from 0 to 4, and another being from 0 to 2. When beating Round 100, or every 100 rounds after that (Round 200, Round 300, etc), the instamonkey rewarded will guarantee one with either Tier-3 or Tier-4 upgrades. It is possible to earn any tower with Tier-5, by clocking the fastest timing of completion in a race event. However, players have to beat the intense competition with every other competitor and demonstrate profeciency in completing the race event in a short time. Alternatively, they can also be obtained through Diamond Chests in Collection Events, or they can be purchased in extremely rare Tier 5 Insta Monkeys Pack sales.
Winning Impoppable or C.H.I.M.P.S. will guarantee earning an Insta-monkey because players get an Insta-monkey when you beat round 100. As of the 3.0 update, a popup comes up upon starting round 101 showing which monkey players got.
If playing on Apopalypse, it is possible, with a long enough map and precise timing, to reach round 100 without popping any bloons or placing any towers due to the nature of the gamemode.
Due to the fact that the insta-monkey is rewarded immediately after Round 100 is 'beaten' the player can simply reach Round 100 and Round 101 will instantly start, netting the player a free insta-monkey, not even having to beat the B.A.D.
However, this no longer works as of version 8.0, as you have to actually beat the B.A.D. in order to get an insta-monkey.
As of the 3.0 update, there is a bug where players can get a infinite amount of insta-monkeys, earning one free working condition instamonkey at a time. However, in order to do so, players must have a round 100 save before the update on any given difficulty, mode or track.
This bug once still worked, but it only worked for Version 2.0 saves.
Tier 5 insta-monkeys can be earned, but can only be obtained by placing 1st in a Race Event, earning Diamond Chests from Collection Events, or from purchasing an extremely rare sale on Tier 5 Instamonkey Packs.