Welcome to Res! Well, with the 0% rarity ones, they're either credit shop pets or seasonals. The credit shop pets (Except for the retired Omni and Kioka now) can be bought with credits that you can get from doing free surveys, buying them with real money ($1 = 10 credits), or buying them with TU (100,000 TU = 1 credit). The seasonals are released for a week around certain holidays. Ebilia/Jaaku = Halloween, Easero = Easter, Iluvu = Valentine's Day, Uilus = Christmas.
Hope you have fun here on Res! ;3