Round trip to bring my dog back.

Hello everyone,

i will be flying to cali in july to pick up my dog so on the flight there itll be me and my bf but on the flight back there'll be an "extra" passenger coming. how do i go about adding her onto the flight back of a round-trip and what do i need to know about flying with her that i dont already know which,

i do know i need her shot records and she needs to be in a kennel if im bringing her as a personal carry on which i will be because i do not trust airport personel to handle her.

Do all airports have an area for pets to go potty?