You see Power Dragon Scale Mail...

[HugoVirtuoso]

I had the idea that PosChengband had all monsters from all variants, seeems strange there's no Emperor Mimic.

Spoiler:
But then, PosChengband does have an insanely-powered level 110 'Plain Gold Ring' monster, not your standard "Emperor Mimic"!!

[Derakon]

Emperor Mimic is from TomeNET, but it's probably based on another variant (it's flagged as BASEANGBAND in TomeNET though).

All that means is that it won't be excluded from the game if you use the birth options to exclude silly or non-thematic monsters (like the Cthulhu/Zelazney monsters).