The mimic octopus can change colour and kind of mimic other creatures, so they’re pretty tough to detect. And in the adventure I like to call The Abyss Stares Back (dun dun DUUNNN!) we discovered a specially evolved one that has even more advanced mimicry skills! I’m surprised we found it at all. I mean, it could look like anything.
There could even be one in this room right now, as I type this.