From Nookipedia, the Animal Crossing wiki
The rare Gar (ガー, Gā) acts as a nighttime counterpart to the Giant Snakehead. It sells for 6,000 Bells, the only fish that does, and is approximately two and a quarter meters long. It appears in summer from 4:00 p.m. to 9:00 a.m.. It is also the most expensive river pool fish.
Donating to the museum
When placed in the museum, the Gar shares a tank with the Arapaima, the middle tank in Animal Crossing: Wild World and the middle-right tank in Animal Crossing: City Folk. Until Animal Crossing: New Leaf, Blathers makes a small speech about the donated Gar.
In Wild World
"<Player>, if you plan to raise a Gar at home, I have some crucial advice. You must be very careful not to overfeed it! These brutes grow much faster than you might suspect, eh wot? Before you know it, it could be bigger than you can handle! The horror!"
In City Folk
Blathers will say this when given the long fish:
"Hoo, I daresay you've really fished a winner this time, my dear owlet! Gar can grow to massive sizes--up to 120 inches! Let's hope this new addition to our museum can grow to an equally enormous size..."
In New Leaf
"Their long faces and large mouths make them look like alligators. Some can grow up to three yards. They are ancient fish that haven't changed since they first appeared millions of years ago. They breathe not only through gills, but also by sticking their mouths above the surface of the water. Gar eggs are poisonous, so people all over the world are advised to not eat them--no exceptions."
Names in other languajes