I'm not under the impression Aquaria would be able to handle realistic tentacles well... most of what I recall seeing from the videos uses more skeletal motion that wouldn't be appropriate for a tentacle. There are a few examples of more fluid motion, like the cape or tails, but I wouldn't be surprised if that were beyond the ability of a mod to replicate.
There's probably a tentacle boss already, but as it was explained to me, "you might think that being a tentacle monster would be good, but it carries a high risk of tenticular cancer." So feel sorry for the guy.