Atrachnacha? I don't think there is any dialogue or scenes referencing her whatsoever. In general, RPGMaker games do this funny thing where most of their assets are generic rather than made for the game specifically, and so often you're left thinking "holy shit why does this guy look like a final boss" and it isn't, and the characters don't even mention it. I think Atrachnacha's sprite is just one of those generic monster ones, and isn't meant to be story-relevant.
Looks like the battler name is just "FOE2", not even mentioning the monster name. I can't seem to reverse-image-search it, though; perhaps it's part of some asset pack.
