Daily Yo-kai Watch Factsさんのイラストまとめ

Bousou Rinne is the first "Rampage" Yo-kai to make a cameo in the non-mobile games, where he appears as a costume for the player to wear in Y School Heroes: Bustlin' School Life.

Although Thurston and Shuten Douji both have the same name in Japanese, and are based on the same real life youkai (also called Shuten Douji), there is no confirmation on whether they are the same being.

Including mobile games, more than 200 "nyan" Yo-kai exist in the whole franchise.

Orifalcon is the first Big Boss originating from Yo-kai Watch Busters 2 to get a variant, that being Jirucondor.

Only 6 'Merican Yo-kai are in Yo-kai Watch 4, those being Usapyon, Cornfused, Silver Lining, Little Charmer, Deadcool, and Rocky Badboya.

Tamayo Daiouji is thought to be the reincarnation of Tamamo due to their similar name and color scheme.

Lord Enma's random nicknames in Yo-kai Watch 3 are Gawko, Veggie, Freezer, and Cody, the first 3 being references to the Dragonball characters Goku, Vegeta, and Frieza.

Junkernaut is the only first boss to not be befriendable in any games.

Submitted by @GGMoman

A carriage known as the "Yo-kai Yamakasa" can rarely be found on the overworld of Yo-kai Watch 4 late at night. They are drawn by numerous oni and drop an item after they pass. (They can also be inspirited and used in ++ Blasters.)

Despite DS Zodiac and DS Ancient being the powered up forms of DS Galaxy, they are Z ranks whereas Galaxy is a ZZ rank in Yo-kai Watch Puni Puni.