Press select to have a hidden wrestler come in the ring. the hidden wrestlers don't get hurt.

Unlock all Wrestlers:
At character select screen press R1, R1, R1, R1, L1, L1, L1, L1, R2, R2, R2, R2, L2, L2, L2, L2, Select.

Cheat Codes
Enter the following codes at the option screen -

Big Head Mode:
R1, R1, R1, R1, R1, R1, R1, R2, Select.

Big Head, Hands, Feet:
R2, R2, R2, R2., R2, R2, R2, R1, Select.

Swelling Head Mode:
L1, L1, L1, L1, L1, L1, L1, L2, Select.

Ring Select:
R1, R2, R1, R2, Select.

All 24 Rings:
L1, L2, R1, R2, L1, L2, R1, R2, Select.

Instant win:
At the character select screen Highlight a wrestler & press L1, R1, L2, R2, L1, R1, L2, R2, Select.