I believe the first thing you need to define for yourself, and also to help those of us who are unfamiliar with the character, is what kind of movement you want to achieve. For example, the costail seems to have three specific characteristics. First the tail moves left and right in a single plane. (it does not move forward and backward, nor does it rotate.) Second, along the starting length of the tail it can be formed to hold a specific shape. (it could be a simple or complex curve in three-dimensions, but it does not change that shape until manually reformed). Third, the end of the tail is loosely connected so that it is largely just under the control of inertia, it continues to move in the same direction until the motion at the base changes (this can be effected by the weight and the speed of the tail). How should your tentacle perform?
Moving the tentacle is probably the easy part. The youtube shows you need an attachment base like the belt harness to which you need to attach a motor. A cam mechanism would probably be the easiest way to set the direction of the motion. The motor would be geared to set the speed of the movement. The costail length seems to be plastic links attached with semi-rigid rods *which can be used to form and hold a curve). The outer edge of the link provides the shape of the tentacle (circular, flat, or irregular, small, medium, or large) These could be 3D printed for maybe even cut from flat plastic sheets. I believe pvc pipe would be too heavy and hard; when it is flipped around, even at slow speed, it could be a hazard to the cosplayer or other guests. Hope this helps.
