The stag beetle is the largest beetle in Europe. These beetles are huge, a male stag beetle with its mandibles is about as long as the palm of my hand.
Even the female has impressive mandibles and one should try to avoid getting bitten by it.
But the stag beetle’s claim to fame is that the males posses huge mandibles resembling deer antlers.
The mandibles are used for display in courtship as well as for fights between males.
Stag beetles are protected over most of Europe as their numbers have declined due to habitat loss (the larvae require specific decomposing wood to feed on). On the other hand at least in Hungary they seem to be bouncing back. When I grew up one always had to go to wooded areas outside the city to see any stag beetles. Nowadays they do appear inside city limits in areas inhabited by humans.
Lucanus cervus on Wikipedia.