Go to the 'Title Screen' and press L1 + R1 + L2 + R2 to enable the cheats screen. Now you can enter the following passwords to activate the corresponding function:
Monkey impersinating Spike from AE1: krops
Monkey impersinating Jimmy from AE2: 2nd man
Red Pipotron: redmon
SAL-1000 monkey: grobyc
SAL-3000 monkey: SAL3000
Blue Pipotron: coolblue
Yellow Pipotron: yellowy
Dark master monkey: blackout