From Nookipedia, the Animal Crossing wiki
The Miyama Stag (ミヤマクワガタ, Miyama Kuwagata) is an insect found in Animal Crossing: City Folk and Animal Crossing: New Leaf. It is a beetle that appears between the months of July and August, and sells for the price of 1,000 Bells.
Donating to the Museum
In City Folk
"With its thick, bumpy, armorlike exoskeleton, the Miyama stag beetle is quite popular with children. However, the truth is, this beetle is fragile enough to perish from injuries we'd usually consider minor. Indeed, it appears this beetle suffers from a case of rather too much bark and too little bite."
It can be found on the back terrace of the insect exhibit, on the third tree from the right.
In New Leaf
"Miyama stags look strong, with their thick, bumpy exoskeletons and yellow hair covering their bodies. They're active during the day but really don't prefer the heat during the summer. Contrary to their appearance, they are rather delicate insects, so be sure to handle with care."
Bug Encyclopedia Information
The Miyama kuwagata is a beetle found in Japan, thus the Japanese name. It is popular among children, because it is seemingly harmless and does not mind being touched. As stated in the "Bugs and Fish" window in City Folk (also known as Encyclopedia), it has a distinctive shape of head, and although it looks strong, it is in fact quite delicate, which maybe why it is popular with children. It is found in mountainous areas in northern Japan, as well as natural forests.
Names in other languages