What animal would have a mouth like that? O.o
Well pinsir's pokedex entry says its the stag beetle Pokemon, so its a stag beetle.
For info on the stag beetle look here: https://en.wikipedia.org/wiki/Stag_beetle
-As you can see from the images on the site, stag beetles have the pinsirs that pinsir has on its head but I the mouth is probably like that just to give pinsir a different design because stag beetles dont have mouths like that.