From Nookipedia, the Animal Crossing wiki
The Scarab Beetle (プラチナコガネ, Purachina kogane) is a rare insect that appears in the Animal Crossing series. It resembles a Fruit Beetle, with only the golden color separating it. It can be sold for 6,000 Bells.
Donating to the museum
In City Folk
"From the gorgeous to the grotesque, the fruit-beetle family has a wide variety of members, indeed... Among them, the shiniest by far is the scarab beetle. Not that it makes any bit of difference to me. If it has antennae, it's most unconscionably vile..."
It can be found on the top terrace of the insect exhibit, on the second cedar trip from the right - sharing it with the Fruit Beetle.
In New Leaf
Upon donation to the museum, the Scarab Beetle can be found on a tree on the right side of the upper right room of the insect exhibit. An information board has this to say about the Scarab Beetle:
"Scarab beetles have elegant, metallic-looking bodies and are shinier even than fruit beetles. They're so shiny that they're said to gleam like gold. They're very rare and can be sold for high prices. Like all beetles in the scarab family, the outer shell provides a very thick layer of protection from predators. The high value and golden shell do make them more vulnerable when it comes to Bell hunters, though."
The Plusiotis resplendens (also known as Golden Scarab Beetle or Jewel Scarab Beetle) gets the name from its elegant, metallic body, which is said to "gleam like gold." They are native to the Americas, unlike the common idea of the location for scarabs, which is Egypt (where they were made famous). The young larva form can be found living in rotten logs, while the adults are found living on foliage. Common colors for jewel scarabs are green, silver, and gold.
Names in other languages