While a tentacle has what could be perceived as a quite complex motion, the construction of an animatronic one is actually fairly simple. There’s a stack of vertibrae that pivot along a center line and cables running along the length to pull and supply tension. You can get more complicated with it, but simple works as well.
There are quite a few tentacle rigs on thingiverse you could simply print and assemble to play with. Not all of them are labelled as tentacles, some are called tails or monster hands, but they’ll all do the job.
Mechanical Tentacle – link
I like this one, the visual design of the tentacle itself is cool enough to show off, it doesn’t even need skin! It also has great motion.
Alien Hand – Link
Labelled as an “alien hand” this is a great example that comes complete with a whole rig to hold everything for operation.
Animatronic Tail – link
This tail would easily pass as a writhing tentacle
Animatronic Tail – Link
Flex fun strip – link
Tiny and amusing. I need a few of these.
Robotic Silicone Tentacle – Link
This one requires a bit more work. You actually print a mold in which to cast silicone that you’ll inflate. Fantastic result though.
Robotic Tentacle Arm – link
Not the most agile tentacle but a fun example complete with servo mountings.
Animatronic Tentacle 2.0 – Link
This one uses an interesting method of creating the flexibility. They used 3d printed spring joints. I’m curious as to how well this actually works, there’s no video.
Simple plate for animatronic – link
Another wriggling tail!
Little tentacle controlled by string – link
This looks like it would probably be a floppy mess, but could give some fantastic wriggling effects.
Modified disk conntectors for tentacle or tail – link
I just liked the way these looked. No idea how well they would work, the cable hole seems almost too close to the center.
Little tentacle controlled by string (with center hole) – link
this is actually v5. You can find all previous versions of this maker’s tentacle rig on thingiverse. The nice part about this design is that the center hole would allow you to route stuff all the way to the end, such as a light or electronics. The channels for cabling seem a little too long here and may encumber motion, check v4 for a possible better motion (but tighter tolerances on the snap fit)
Simple tentacle vertebrae – link
extremely simple, would work on even the cruddiest 3d printer.
You can find a few more on thingiverse that are basic variations of the vertebra. There are also many that are simply poseable, not animatronic, so I didn’t include them. Maybe we’ll do those at another time!