Saw stag

The Saw Stag (ノコギリクワガタ, Nokugiri Kuwagata), known as the Saw Stag Beetle prior to is a beetle that appears in all  games. It can be sold for 2,000 Bells.

In Animal Crossing
"They're called beetles, wot? The lugs of the insect world. Insects with armor plating on their backs, you know. Their armor, it...opens up, revealing wings, which they use to fly. It's really quite unnatural. Then, there's that thin membrane-like covering where their heads connect to their bodies. Most foul!"

In City Folk
Blathers will comment on the donation to the museum as follows:

"The saw stag beetle's most distinguishing feature is its large, curved jaw, wot? The larger the specimen, the more teeth in its jaw. Among collectors, the teeth settle a specimen's worth. I don't suppose bugs have dentists... Hoo, I'm quite sure that would be the worst job of all time."

After donation, the Saw Stag Beetle can be found on the top tier of the insect section, on the fourth tree from the lantern on the left side.

Further information
The Prosopocoilus inclinatus (Saw Stag Beetle) is one of the most common Japanese stag beetles. They are kept as pets in Japan, much like many other beetles. The mandibles on its head change shape as the beetle grows - the larger it becomes, the greater the angle and size the mandibles become.