There are a few beetles that are can easily be mistaken for a stag beetle.
The lesser stag beetle (Dorcus parallelopipedus)
At first glance the lesser stag beetle looks almost identical to a female stag beetle. There are a few subtle differences though. The lesser stag beetle is normally smaller than a stag beetle measuring up to approximately 2.5cm (1″). The wing cases are darker and rougher than those of the stag beetle and they have a slightly flatter profile.
Neither the male or female lesser stag beetle have the large antler-like mandibles (jaws) of the stag beetle. The male lesser stag has a slightly broader head and slightly larger mandibles than the female. However, the best way of telling the male and female lesser stag beetle apart is by looking at the top of their head. The female has two small bumps that are absent on the male.
Cockchafer (Melolontha melolontha)
Cockchafers or Maybugs, as they are also known, are much hairier and have much lighter, almost chestnut brown wingcases than the stag beetle. They are also generally a bit smaller, measuring up to approximately 3cm (just over 1″).
Unlike stag beetles they are attracted to light and will often fly into windows in the evening.