Daily Yo-kai Watch Factsさんのイラストまとめ

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.

6 Deva Yo-kai were added in Yo-kai Watch Blasters 2, those being Yami Enma, Fudoumyouou, and the 4 previous Lord Enmas.

Steve Jaws' toy medal received a voice change after the Japanese DX Yo-kai Watch Model Zero in the DX Yo-kai Watch Model U and onward for seemingly no reason.
Model Zero audio clip (0:11) : https://t.co/nkdVnQbtG6
Model U audio clip (0:36) : https://t.co/cCZ5EMbQw7

Bronzlow is the main villain of the Yo-kai Watch Shadowside manga. His presence solves many plotholes in the original Shadowside series, for example explaining how the Yo-kai lost their memory and how their physical forms changed over the 30 years.

Jabow is a snake Yo-kai from Yo-kai Watch 4. He's actually the mascot of Omuta city in Japan's Fukuoka prefecture and was designed by Level 5!