I will go first. The first thing that comes to my mind is the
“Boxfish”
Also know as cofferfishes, cowfishes and trunkfishes. “The hexagonal plate-like scales of these fish are fused together into a solid, triangular, box-like carapace, from which the fins and tail protrude. Young boxfishes have a more rounded in shape and may exhibit bright colors. Because of their body scale structure, boxfishes are limited to slow movements. Boxfish are edible, and are sometimes baked in their own shells. In the aquaria, boxfish generally are very personable fish yet are poorly suited for the home aquaria. They are not thought of as an ordinary aquarium tank mate, but are quickly gaining popularity. They do pose a hazard in the community tank however. They are capable of releasing a very powerful toxin which can kill other fish and in some cases, the boxfish itself. They generally only use it when threatened or dying, but can become disturbed easily with aggressive tank mates or overcrowded aquarium” (Wikipedia).
