Legends of "Rakshasa" have been around for centuries. Mostly, I believe, in Hindu and Buddhist lore. I've also heard that in Malaysia, their word for "monster" is a very close derivative of the word.
That's the old pen-and-paper Dungeons & Dragons geek I keep inside. Sometimes he sees something familiar and reminisces the days when I let him have full reign. He clamors, and I must let him out for a few minutes so he doesn't soil the carpet.
Anyway, that's kind of interesting. Koontz seems to have an interest in Hindu lore - I know one of his Odd Thomas books focused heavily on their death god, Kali.