This is a gynandromorph (an individual that consists of male and female) of Allotopus rosenbergi (stag beetle species living in Malaysia and Indonesia). The body has bilateral asymmetry, half side female and the other side male.

This phenotype is quite rare, but have been widely observed in arthropods both in nature and in breeding conditions. We can see gynandromorphs of other stag beetle species as well on the internet (Ref.1, the link shows the gynandromorph of Dorcus hopei binodulosus, Prosopocoilus inclinatus inclinatus, Lucanus maculifemoratus maculifemoratus and Dorcus titanus castanicolor ).

What type of developmental mechanism makes this phenomena? The review by Narita et. al. (Ref.2) describes it. According to their review, several types of developmental abnormality at early developmental stage (typically the single or two-cell stage) disturb the normal distribution of sex chromosome to the daughter cells. Because the sex chromosome determines male/female phenotype, the distribution abnormality allows female cells and male cells to coexist in an individual. The bilateral asymmetry suggests the developmental mechanism in which once cell in the two-cell stage makes half of the body and the other makes the opposite side.

Reference:
1. Nagasaki bio park’s blog
2. Narita, Satoko, et al. “Gynandromorphs and intersexes: potential to understand the mechanism of sex determination in arthropods.” Terrestrial Arthropod Reviews 3.1 (2010): 63-96.