Tentacled horrors have become one of the defining characteristics of the Mythos (despite the relative lack of tentacles in Lovecraft's work). Thanks to that established trope, one of the easiest ways to "Mythos-ize" a prop or found object is to add a few tentacles using Sculpey or epoxy putty. Luckily, the Polymer Clay Fan website has an in-depth tutorial on sculpting realistic tentacles.
If you've ever thought of sculpting your own Cthulhu idol I highly recommend giving it a read. It seems counter-intuitive, but for a beginning sculptor this technique is easier and will produce better results than trying to create smooth tentacles. A flawlessly regular and unbroken finish is fiendishly difficult and takes quite a bit of skill to pull off. Adding texture lets you cover up a multitude of errors and produces a more visually interesting result.