See endings:
At Title screen, press L2, R1, L1, R2, L2, R1, L1, R2, R2.All monsters:
At Monster Ranch highlight "Buy Monster" and press R2, R1, L2, L1, R1, R2, L1, L2, L2. You'll hear a sound to confirm.
Thanks to Revolution readers Frost & Death Charge, David Smith, and Robert Case!