Many of the cheats for the Cats and Dogs Expansion Pack require TestingCheats to be enabled. Please see that page for more information. As with all cheats, it may help to keep the game paused throughout this process.
Cats and Dogs Skill Cheats
Two cheats exist to directly set the level of a human Sim's Veterinary and Training skills. With Testing Cheats enabled, make active the Sim you wish to change, then open the cheat console again for the following codes.
Veterinary Clinic Perk Cheats
These cheats add the perk to the current family's vet clinic. Any Sim from the household needs to be at the lot you are affecting; multiple veterinary clinics owned by the same family are handled separately. There is no way to remove a perk you own.
Additional Animal Edits
With TestingCheats enabled, you can shift-click your pets to add or remove personalities such as what appliances or other objects your pet fears or loves. You can also shift-click your pet to fully edit the animal in Create a Pet in case you want to change their gender or Pet Traits. Breed and body changes can be done in this mode as well, but they can also be done without cheats if a human Sim interacts with a mirror and uses the "Change Sim" command since the entire family (animals included) will be taken to the Create a Sim screen.