"Look! I found an actually-harmless creature from the deathworld. It's so cute. It doesn't even have a mouth to bite with. I'm taking it home as a pet."
"That's a box jellyfish." -- Anon Guest
The creature in the tank pulsed rhythmically and didn't look dangerous at all. It seemed no more dangerous than a plastic bag. That statement alone should tell everything. Even the most harmless-seeming things from a Deathworld have hidden menace.
"See those tails? The tentacles?" said Human Di. "They're venomous. Deadliest stingers in the world. These jellyfish drift around until a tentacle comes into contact with something and then they tangle it up and eat it. If the prey's too big? They kill it anyway. That includes big mammals like me."
Companion Korrik backed away from the tank. "Oh." They were lucky they hadn't tried to pet it. "I was fortunate that I used devices to collect this."