Pinsir is based on a Stag Beetle. If you want a specific species, then it is likely something along the lines of Prosopocoilus inclinatus. This is a popular beetle used in insect fighting.
This supported by the appearance of its regular form, with the large pincers on its head resembling those seen in the photo. This is also supported by the mega form, which adds wings that are extremely similar to those seen by many beetles, including the ones in the picture.
The fact they are used in insect fighting also explains why Pinsir learns so many Fighting type moves levelling up (Revenge, Seismic Toss, Brick Break, Superpower, etc.) despite the fact it is not actually a Fighting type itself.