I hadn’t even noticed this until a commenter pointed out that the name “Pokemon” must be based on borrowed words (namely “pocket” and “monster”) because all words in Japanese with an initial /p/ are either borrowings or onomatopoeia. A quick bit of research revealed that the reason for this strange gap is that what is now /h/ (ish) was once pronounced /p/ (ish), and all the words starting with /p/ drifted to the present sound, leaving us initial-p-less (drifting of sounds like this being a normal linguistic process). I haven’t found out why /p/ does exist in the middle of Japanese words, but I imagine it’s because a middle /h/ is so damn hard to pronounce.
Why do Japanese words not start with p?
October 24, 2015 at 1:19 pm (Japanese history, Japanese language)
Japan Explained FASAQ 
noah said,
March 14, 2016 at 3:35 am
Basically, I think /p/ in the middle of Japanese words (except for onomatopoeias, loanwords, and modern coinages) is generally doubled or after /n/ (/m/), which are places where I guess it would be more difficult for it to soften into /f/ or /h/.
Jeffrey Huffman said,
March 30, 2017 at 11:40 pm
Pontocho.
http://www.japan-guide.com/e/e3921.html
You’re welcome.
alexcase said,
March 31, 2017 at 12:13 am
Interesting, although it’s a place name rather than an actual word, and given that the pron doesn’t really match the kanji of its name it suggests that it comes from local dialect or is even pre-Japanese in origin. Thinking about it further, there are also of course sounds that start with “p” in Japanese, e.g. “paku paku” for eating, made famous as part of the name of PacMan.