Why is Bonk called PC Genjin in Japan?

Say it in Japanese and the joke lands at once. PC Genjin, PC Caveman, is pronounced almost exactly like PC Engine, the console Bonk was created to sell. Genjin means primitive man or caveman, which is what Bonk is, and the PC ties him permanently to his machine: he is not just a mascot on the PC Engine, he is the PC Engine, prehistoric division. The character began as a comic in Gekkan PC Engine magazine, so the pun was his name from birth, before any game existed.

Hudson built a second joke under the first. The PC in his name officially expands to Pithecanthropus Computerurus, a mock scientific species name modeled on Pithecanthropus erectus, as if Bonk were a fossil species adapted to computers. Europe kept the system reference and dropped the caveman word, calling him PC Kid, and some European versions used BC Kid instead. America dropped the whole apparatus and named him for his attack: Bonk, the sound of the forehead meeting the enemy. The British market is part of why the American name never crossed back, since bonk carries a rude slang meaning in British English, a point contemporary British reviewers of the Amiga version made while praising the game.

The system held for every move he made. Ported to the Famicom, he became FC Genjin, with FC now standing for Freakthoropus Computerurus. On Game Boy he was GB Genjin. On the Super Famicom, Super Genjin. No other mascot of the era renames himself after each host machine, and none needed to: the name was never really his. It was the console's, lent to a caveman.