Daily Yo-kai Watch Factsさんのイラストまとめ

Tigappa's key art is a rare instance where despite being a palette swap they have completely separate artwork from their original counterpart.

Mamemameinu, Miirakansu, and Nego are Onryo that made their first appearance in the Y Academy manga, later appearing in a Y Academy Gates of Whimsy event in Yo-kai Watch Puni Puni. All are fanmade characters from children in a Corocoro Comic contest.

These images of Gilgaros, Komashura, and Dandoodle show how the artist for Yo-kai Watch, Takuzou Nagano colors in his artwork.

Rocky Badboya is based off of Joe Yabuki from the "Ashita no Joe" series and could also possibly be based off of Glass Joe from the "Punch-Out!!" series due to his weak personality and nationality.

The Yo-kai Watch Style Guide has a section stating to "not use Yo-kai with other IP", showing a silhouette of the Pokémon Pikachu next to Jibanyan.

Beta designs for Wampus and Momo are used in the Y Academy manga, where Wampus has 2 wisp tails like a "nyan" Yo-kai and Momo is less round with smaller eyes. Wampus' beta artwork hasn't been found but the author of the manga Noriyuki Konishi accidentally used beta Momo as (1/2)

The first Yo-kai you fight and defeat in the befriending tutorial in Yo-kai Watch 1 automatically becomes your friend. This can allow you to even 100% befriend Snartle at the beginning of the game by exploiting the Yo-kai Cam.

The Jawsome Kid's given name is actually "Paul Jaws" according to Steve Jaws in Yo-kai Watch 3. His name is Paul in both Japanese and English versions of the game.

Earth Walker's beta form was accidentally revealed in Yo-kai Watch Puni Puni, where he seems to have a tighter jacket, a normal YSP Watch instead of a UR Watch, and slightly bigger zig zags on his jacket.