To race on sheep, go to the transmission select screen and press:
Up, Up, Down, Down, Left, Right, Z, Y.

You'll hear a noise to let you know it's worked.

To get superbikes go to race options screen and press L+R+X+Y+Z at the 
same time. The superbike option should appear on bike select.

You can combine both these options.

Secret Tantalus Bike To find this hidden extra bike bearing the livery 
of the developers:

Choose Arcade Mode. On the transmission select screen enter:

Up, Down, Up, Down, Left, Right, Y, Z, and quickly choose your 
transmission.

To get a great start, accelerate with your brakes on, to 
keep revs about 10. When the voice says 'go!' let the brakes off. A 
voice should say 'Great start!' as you scream off.