The Sims 2 Pets (PS2 version)

Unlock all breeds:
While choosing your breed, press up, up, down, left, left, right, circle, x, x, triangle.

Short bonus movie
At the EA loading screen, press x, circle, triangle. Now the opening movie will be replaced with a shorter movie about cats and dogs. 
   
Don't pay your bills
When a bill arrives, pick it up and drop it. Now go on grab mode, pick it up and delete it - and it's gone! No questions asked. Sadly this doesn't work on real life bills. If only...

Cheat Gnome (needed for other cheat codes to work):
At The EA logo screen press L1, L1, R1, x, x, up.

Max all motives
During gameplay press x, x, triangle, square, square, down, down, left, right, right. Now click on the gnome, and click on 'Max all motives' now all of your motives are restored.

Get a horse as a pet
At the breed selection screen press L1, R1, x, circle, square, R1, R1, x, x.

Extra money
Pause the game and press: right, right, left, left, right, left, right.

Sadly these other cheats won't help you take care of real animals.