These are the Sailorettes' (inners) cats. These can talk and also instrumental to them. I knew they once transformed into humans, but the Filipino series skipped the Sailormoon S episodes where Luna and Artemis turn into humans. If you know what these cats did as humans, tell me.

This is Bunny's (Usagi) magical black cat who gave her powers as Sailormoon. And according to Sailor July Moon (sailorsmurf2000@hotmail.com), she's the only one that turned into a human. Here's what she said:
She was falling in love with a human named Kakeru and her wish was to become human to help Kakeru fight the evil crystal draining his energy. Sailor Moon used the Silver Crystal to grant her wish.

This is Petunia's (Minako) magical white cat who first gave her her powers as Sailor V, then gave her powers and became her adversery as Sailorvenus.

This is CB's (Chibiusa) magical lavander (anyway, looks more lavander than pink) cat who came from the 30th century. She is Luna and Artemis's little kitten at that time. (Just like her little mistress with Usagi and Mamoru (or Neo Queen Serenity and Neo King Endymion.). She only showed up since Sailormoon SuperS.
Luna: ©MCMXCII Naoko Takeuchi / Toei-Kodansha / TV Asahi
Artemis: ©MCMXCI Naoko Takeuchi / Toei-Kodansha / TV Asahi
Diana: ©MCMXCV Naoko Takeuchi / Toei-Kodansha / TV Asahi