"Human-loving, fast-talking, street-smart, SLAM-dancing...Moogle..."
Mog is a playable character in Final Fantasy VI (original title), a.k.a. Final Fantasy III (NA release), who appears multiple times throughout the storyline and eventually becomes a permanet ally. He is also depicted on the box art despite his relativly small role in the game's plot. In Mog's defense, without the help of Mog and his fellow Moogles, Terra and Locke may have never escaped from Narshe.
Mog is a Moogle who lives in the mines of Narshe along with the rest of his species. Mog is unique from his peers due to his ability to speak human language (taught to him by Ramuh - an Esper - in a dream) where other Moogles would only say "Kupo" or a variation of the word. Additionally, he has the special ability to dance that results in one of four random attacks based on the dance. If his dance attack fails, he falls flat on his face. Mog also has the highest defense compared to other playable characters.
Mog is not the first of his kind to appear in the Final Fantasy series. Moogles first appeared in Final Fantasy III. Moogles have since been included into every subsequent Final Fantasy game (excluding Final Fantasy IV) with variations to their visual design.