Blowfish

The blowfish (フグ, Fugu) is a fish introduced in that can be found in the ocean between November and February. It can be sold for 125 Bells.

In New Leaf
Upon donation of the blowfish, the information board outside the tank that houses the blowfish says the following:

"Blowfish are considered a delicacy, famous for their tempting taste despite being highly poisonous. Blowfish are not poisonous at birth, though, which makes young ones relatively safe for consumption. It's only once they have ingested enough microbes from seawater and food that they become poisonous. When they feel threatened, they suck in air and water to blow themselves up like balloons. This makes them too big to eat without predators being forced to chomp down on their poison quills."