I wouldn't quite go that far, but I'm used to Cthulhoid horrors

I found it a little odd, but interesting. Now if the arm takes over your mind, all bets are off. The nice thing about it is that the mechanism is fairly simple and therefore not as expensive as some. But it might unnerve those with low sanity points. :)

Tentacle-Like Prosthetic Arm Will Haunt Your Dreams

Kaylene Kau’s prosthetic arm is either a sweet modern update of the old-fashioned pirate’s hook, or a terrifying device that will turn its wearer into a Cthuloid-human mongrel. I favor the former, if only because I want to sleep at night.

The prosthesis is not designed to be a prehensile limb, but instead it’s an assistive appendage for the good arm. A simple motor drives two cables inside the tentacle, and the wearer controls it with a pair of switches on the upper section. Just put the “arm” in place, hit the switch and it curls around whatever you might want to carry. The other switch unfurls the arm.