In Palace Pets other characters and pets have appeared that don't aren't officially affeliated to the Princess' or considered part of the merchandising in this series.
Most of them appear in stories of the pets or Whisker Haven.
Beasty and Bouncy
In Beauty's original book, it was revealed that she has twin brothers; a pink-purple kitten named Beasty and a red kitten named Bouncy. Beasty has a neatly brushed tail and bangs of dark pink with a peach-pink body and white accents. While Bouncy had a colorscheme matching a fox and had messier hair and tail.
In Pumpkin's book, it has revealed that Pumpkin had 4 sisters.