Stag beetles are a personal favourite of mine, and that of many beetle enthusiasts. I think this must be because they look so very impressive, and because they are one of the largest beetles commonly seen in the UK. To look at them, you would think that they would have very few enemies if any at all, but believe me when I say that they are much less frightening than they seem.
Let's start with the 'antlers' which are basically hugely enlarged jaws, and only present in the males of some species; the jaws of the female are much smaller. The antlers are used for 'wrestling' with other males over mating and territorial disputes and, because of their length, cannot deliver much of a bite at all. Secondly, the diet of the stag beetle is chiefly made up of decaying wood and leaf material. Stag beetles can be considered to be gentle giants.
Stag beetles have their share of natural predators and enemies, and when you used the word 'natural' I presume you mean enemies other than man, who seems to spoil the party for most other animals on the planet these days. Since stag beetles don't really have to compete for food to any appreciable degree, we can break down their enemies into just parasites and predators.
Many beetles carry mites, in fact the Dor Beetle is so renowned for its mites that it has the nickname 'The Lousy Watchman'. Now whereas most species of mite won't cause the beetle enough harm to affect its lifestyle at all (some could even be considered as helpful), there are a few who's larvae and behavior can do the beetle enough harm to kill it.
Predators are chiefly mammals, reptiles and birds because of the size of the beetle. Hedgehogs, foxes, stoats, rats, lizards and a host of other animals will happily dine upon a stag beetle - my cat was always adept at bringing them into the kitchen to eat in comfort. I cannot think of any insect predators that would attack a stag beetle with the sole intention of eating it, except perhaps if the beetle was unfortunate enough to cross a marauding swarm of ants. This, of course, does not happen in the UK because our ants are neither large nor aggressive enough.
I would like to finish my answer by sending you a picture of a huge Indian Stag Beetle, and leave you to think of the size of the jar you would need to take him into school!
Have fun,
Justin Roux.