Daily Yo-kai Watch Factsさんのイラストまとめ

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.

Pakkun's Soultimate Move in Yo-kai Watch Puni Puni is a reference to the Super Mario character Piranha Plant. The Soultimate Move is called "パックンフワワ" (Pakkun Fuwawa) and Piranha Plant's Japanese name is "パックンフラワー" (Pakkun Furawa).

Whismellowman is a Yo-kai exclusive to Japanese and Korean versions of Yo-kai Watch Blasters (and Yo-kai Watch Puni Puni). He was changed into the Yo-kai "Whisped Cream" in other localized versions, likely to avoid copyright issues.

Mad Kappa and Badsmella are the only Classic Yo-kai that do not get a rank up after being inspirited by a Wicked.