This sea slug steals jellyfish stingers and fires them as its own weapon

Some nudibranchs eat stinging animals like hydroids and jellyfish, then pass the undischarged stinging cells through their own gut undamaged, storing them in special sacs on their skin. These borrowed weapons, called kleptocnidae, go on to defend the slug exactly as they once defended its prey.

— Wikipedia contributors, Nudibranch — Biological record