Breeding your Sims 3 pets
Even though I bought the Pets expansion pack when it first came out, I have only barely done any breeding of my pets until recently. Sure I bred a horse or two, but nothing like the concerted effort I have been making lately to create my own hideous Frankenstein-like dog breed. (I think ugly things are more interesting than pretty things!)
Elder pets
One thing I have learned is that - at least in my experience so far - elder pets CAN get pregnant. Unlike human Sims where females cannot get pregnant after they become elders. I had two elder dogs, bred them together, and the elder female carried two puppies to term. Maybe that's the developers nudging the scales in our favor, since pets have such relatively short lives compared to humans.
Cheat up their relationship
The easiest way to breed two pets is to use a cheat to boost their relationship status. (Even though I rarely cheat otherwise, I almost always do this for my pets. I just don't have the time to have one dog woo another dog. Get real!) Type ctrl + shift + c to bring up the cheats window, then type in:
testingcheatsenabled true
Hit Enter to close the window, and go back to your game. Go to the Relationship tab on your pet's menu, and find their partner. You can click to drag the relationship bar all the way into the green.
Once you have their relationship up, all you need to do is buy a pet house (where the Woo Hoo will happen), then click on the other pet in order to try for a litter.
Adopting out
Although you can sell horses at the Equestrian Center, you may be wondering how to get rid of any unwanted dogs or cats from your breeding program. After all, your house will quickly become overrun with pets in all life stages if you don't start culling your stock.
The answer is to have your Sim use their phone to put a pet up for adoption. You get to choose which pets to adopt out from a menu. Within 24 hours, a random Sim will show up at your door and adopt the chosen pet. Luckily, unlike in the real world, other Sims don't seem to care if the pet is elder, a puppy, a cat, a dog, whatever - someone always comes to claim it.