Get the good ending
Capture all 300 monkeys to get the good ending.
Unlock the final boss
After collecting all 300 monkeys in the game, you will unlock the true final boss.
Go to Simian Citadel after you’ve beaten the game and find the magician monkey by the fountain. After he summons his mouse thieves, let them steal your money. Once they’ve done this, hit them and they will drop your coins. These coins will be worth 5X and 10X the normal coins and you can continually do this.
Complete the game after finding all 297 monkeys. Go to the main menu and highlight New Game, then press L1 + Start to play as Spike.