it's not in the retired section or anything, but I just got an Experiment 164 pet off of the zapper and can't seem to activate the achievement. I've visited my Pets page, gone to its profile, refreshed the achievement section before and after, the works, but no luck. If this achievement can no longer be acquired, can it please be moved into the 'retired' section or otherwise marked as no longer applicable?

it's inherently retired by being in the legacy section. Legacy achievements are from before the current achievement system and are no longer obtainable.

Ah, :/ Thank you for letting me know. I'll ask this to be deleted and moved. I was so happy for the zap, too!