Answer:
Uhh... Pets and Sims can have baby's.(I'm doing for sims 3)
Anyway, to have a SIM baby, you need a male and female or female and female that are romantic interests or married or fiance
You also need a bed and a high relation ship level.
For cats to have babys, you need a dog kennel and a female and male with high relationship levels.(same for dogs)
For horses to have baby's, you need a male and female and have them at a high relationship level. You also need a stable.
How to find out they are pregnant:
You will hear a lullaby.
And if you cheat - enter testingcheatsenabled true when you open the cheat box (ctrl c) shift click on them and if there is not an "Edit Sim in Create A Sim" it means they are pregnant
Warning - Cheating doesn't work for unicorns