Press Start + Up at the wrestler selection screen.
Pause game play and press Down(4), L. The code may be entered again to return to normal.
Wrestle as Mankind:
Press L, R, A, Down, Left, A, Down at the Acclaim logo.
Wrestle as Mr. Perfect:
Press Down, Left, A, B, C, L, A, R, Down at the Acclaim logo.
Wrestle as Stone Cold:
Press L(2), R(2), A, C(2), R, Down, Left, L at the Acclaim logo.