@HayesKerrI don't know why I did not think of this yesterday but there is a much easier way than using testingcheats. Something you can try first is adding a pet through Manage Households.
1) Go into Manage Worlds from the button at the top right of your screen and then select Manage Households
2) In Manage Households find your household and select it
3) For your household select the Edit, add, or remove Sims from the household in Create a Sims button in the lower right (the icon looks like a pencil)
4) From there click on the Add a Sim button in the lower left corner. With this you can add a new pet to your household just like as if you were starting a new game. You can make the pet look any way you like, choose traits, age, gender, give it a name, etc. When you are done, just to back to live play and the pet will be part of your household.
If you've already adopted a new pet you'd like to change to look like your old pet, we can still do that with the testingcheats. Let me know if you still want more information on testingcheats and CAS, ok?