Build a simple low poly 3d model and pixel over it.
I recommend 3dsMax, but there's many other alternatives.
Or take a photo of a paper cube or a dude made of playdo from above with your phone.
And if you're not *that* hands-on with your stuff, you can still use the 'S' idea (which I still highly suggest doing) but emphasize the bottom-half of the 'S' a bit less than I did in my edit. A straight line spine just makes your character look boring and completely unlike a 'tentacle' since people generally identify tentacles as wavy-wobbly tendrils, which emphasizes the 'organic' aspect of them I'm assuming you want. Emphasizing at least a slight curve, even from the front, helps a lot with this.
The biggest issue I had with making the form for my edit was your palette. Tweak that first. I had to tweak it a bit so that I could express the forms better.
Also there's no hueshifting in your sprite. I added some in my edit to make it a bit more interesting to look at. The difference is almost not noticeable -- but it really is.