Puffer fish

A puffer fish displays its "perfect defence". (TV: Listen)

You may be looking for the humanoid Blowfish.

A puffer fish, or blowfish, was a type of fish which the Twelfth Doctor observed as an example of "perfect defence". It had the ability to increase its size and protect itself from predators with spike-like spines. (TV: Listen)

Puffer fish naturally produced a venom called tetrodotoxin. (PROSE: Cat's Cradle: Warhead) Gilles Lemaitre captured puffer fish for the tetrodotoxin he needed to create zombies. (PROSE: White Darkness) The Sixth Doctor compared the Mykosians to puffer fish in that both produced substances which could control the minds of others. (PROSE: Mardi Gras Massacre)

The Eighth Doctor speculated that Jack Dupre would serve poisonous Japanese pufferfish for refreshments. (PROSE: The City of the Dead)

The London Zoo kept puffer fish in their aquarium. (PROSE: Stanley)