It's expensive to use animals for TV shows or movies, plus, they're unpredictable (even trained animals). With Days having the "one take rule", they don't want to have animals messing up a scene, necessitating multiple takes. Same with babies or little kids.
Pookie (Nicole's dog) died. Back when Vivian had the sarcophagus made to hold Maggie captive, she had Isabella's remains moved to a pet cemetery (telling Victor the elaborate sarcophagus was for Isabella). During the pet cemetery scenes, we saw Nicole at Pookie's grave.