Mewnium Z appears in 3 version groups since Generation VII, an Unused item in the Key Items pocket
PokéAPI item 965 of 2,223, Unused category, Key Items pocket
Effect, in PokéAPI's words
Mewnium Z: “XXX new effect for mewnium-z--bag”
Games
Mewnium Z is listed in the text of 3 version groups, first in Generation VII
Game index by generation
IIIIIIIVVVIVIIVIIIIX
Named in
Ultra Sun and Ultra MoonLet's Go Pikachu and Let's Go EeveeSword and Shield