Puffer fish

The Puffer Fish (ハリセンボン, Harisenbon) is a saltwater fish in the introduced in.

In
Upon donating the Puffer Fish to Blathers in the museum, he will say this:

In
As the Puffer Fish is donated to the museum, Blathers will say this:

It can be found in the back sea tank, with other sea fish. It spends time as the non-puffed fish, then will puff up and show its spines when another fish comes too close.

In
An information board in the aquarium displays this description of the puffer fish upon donation:

Real-world information
As a defense mechanism, puffers have the ability to inflate rapidly, filling their extremely elastic stomachs with water (or air when outside the water) until they are almost spherical. Thus, a predator stalking the puffers may suddenly find itself facing what seems to be a much larger fish and pause, giving the puffers an opportunity to retreat to safety. When the fish is lifted out of water, there is a risk that puffers may inflate with air. This may result in problems deflating again afterward. When this happens with aquarium specimens, the recommended course of action for fish keepers is to hold the puffer underwater by the tail, head upwards, and shake the fish gently until the air escapes out of the mouth.