Pufferfish is a mob in the 1.13 Update (Like the cod, tropical fish, and salmon) they spawn in Lukewarm and Warm Oceans. They will poison the player whenever they get near. if you kill them, you get a pufferfish. eating pufferfish gives you poison and nausea. They have 3 states, Normal, Semi-Inflated, and Inflated. The inflated state can cause the poison.
The 3 pufferfish stages