Breeding

To have more little bundles of fur running around the house, consider breeding pets. Keep in mind that each pet will take up a family slot. In The Sims: Unleashed, a lot can only have a total of eight pets and Sims in one house. In The Sims 2: Pets, a lot can have a total of ten Sims and pets, but no more than eight Sims and no more than six pets. Obviously, the basic thing needed for breeding is two pets of the same species and different gender. However, they can't just be made to breed on command. Just like Sims, they will need to get to know each other first. If they misbehave with one another, use 'Scold' so they learn to get along.

The Sims: Unleashed
In The Sims: Unleashed, when two pets of the same species and opposite sexes perform the "snuggle" interaction, the player will get a message asking whether or not to go through with the breeding process. Just like having a Sim baby, a new item appears; a "maternity tent", which the mother will regularly check on if she's in a good mood. The player doesn't have to take care of the puppy or kitten (although they can view the tent), but they do have to take care of the mother, since her mood will reflect on her baby. Unhappy mothers will neglect the puppy or kitten, in which case Animal Control will come to take it away. If the puppy or kitten is not taken away, it will become an adult after three days. At that time, the player must choose whether the family will keep the new pet or give it up. If the player chooses to keep it, the new adult will appear, and can be named. Otherwise, if the player chooses to give it up, it will disappear from the game, and the family will get some cash. The cash earned can be anything from §250 for two parents with very low skills to §3,000 for parents with maximum skills. A kitten or puppy produced by breeding will have no personality points.

The exact formula is: $$P = \frac{( m_h + m_t + m_o + f_h + f_t + f_o )}{2}$$

When $$P >= 250$$

Where
 * $$P =$$ profit in Simoleons
 * $$m_h =$$ mother's hunting skill
 * $$m_t =$$ mother's tricks skill
 * $$m_o =$$ mother's obedience skill
 * $$f_h =$$ father's hunting skill
 * $$f_t =$$ father's tricks skill
 * $$f_o =$$ father's obedience skill

Note than the skills are on the 0-1000 scale used in the code, not the 0-10 scale displayed in-game. If $$P < 250$$ then the profit made is set to 250.

The Sims 2: Pets
In The Sims 2: Pets, a pet house is required. This allows the player to have the selected Sim command a pet to try to have puppies/kittens with another pet. The Sim must have a good relationship with the selected pet to successfully command it. Additionally, the two pets must also have a good relationship with one another. If successful, the two pets will go into a pet house and WooHoo. If the female pet becomes pregnant, the player will hear the same music that plays when a Sim conceives.

Pregnant female pets show little noticeable signs of pregnancy, and can go to work as normal. However they do suffer from the same decrease in needs as a pregnant Sim. Like Sims, they give birth after three days. When the pet is about to give birth, a sparkle and glow surrounds her. A pregnant pet can give birth to anywhere from 1 to 4 puppies or kittens, though the maximum size of a litter depends on how crowded the Sim household is. (A household cannot contain more than 6 pets or more than 10 members.)

The player has the option of keeping a pet's offspring, giving them up for adoption or selling them. The simoleons earned by selling the litter is based on how well trained the pet is. More training equals more simoleons!

Note: Due to a Maxis bug, pet pregnancy controllers are not deleted when a pet gives birth. Mods designed to fix this problem are available at Simbology and Mod The Sims.

Notes & Tips

 * If you want your pets to have a better relationship, lock them in a room with two food bowls and two beds, and two litterboxes. Don't put any toys in the room, so when the pets get bored they will be forced to play with each other.
 * You may breed a large dog and a small dog, but the puppies will have only their mother's genetics, and will be the same size as their mother.
 * Another way to increase the relationship between two pets is to individually praise them whenever they are playful or playful towards others. Eventually when the pet learns to be playful, reached when the skill bubble pops, a Sim can command them to "play with" and select any pet on the lot. Having the pets play with one another increases the relationship.

Dogs and Cats
Dogs and Cats can have litters like in The Sims 2. Pets are controllable, and with a high relationship, can be controlled to use the pet house to woohoo. If a litter is conceived, lullaby-like music will play. If it does not play, then another attempt is needed. When the pregnant cat/dog is about to give birth, there is actually no need to go to a hospital/vet. Unlike "The Sims 2" the cat/dog does not glow when it gives birth.

Horses and Unicorns
Horses can try for one foal at a time. A stall box is needed for mating to occur. Both horses must have a high relationship with each other, and after socializing for a while, a 'Romantic...' tab will appear in the action menu. Leading a mare to the Equestrian Center is possible if there is not a stallion to breed with. A horse can also breed with a unicorn and produce a unicorn/horse hybrid.

The Sims 4: Cats & Dogs
Breeding returns in The Sims 4: Cats & Dogs.

Breeding
Dogs and cats can have litters of 1-3 offspring, depending on space in the household and randomness. To breed two animals, you must first become friends with one of them. After that, the option 'Encourage to Mate...' will show up and that will allow you to choose another pet to breed your pet with.

About a day after the pets breed, a notification may pop up saying that your cat/dog is expecting.

Birth
After about 2 days, a notification will show up saying that your pet has gone into labor. A little bit after that, the pet will give birth on its own, and a screen like the one for a sim birth will show up, which will allow you to name the offspring