This is hard.

I think i had quite a good time with the bouncing ball...it was a fairly easy task. Though when adding a tail, there's quite a bit more to concentrate on. I've made a quick storyboard, with some notes on the character, how i'd like it to act and a few things to focus on when animating.

I started by animating the ball on it's own, after having a good idea of what i wanted the character to do, and to act like. Afterwards, and after tweaking the animations on the ball, i added the tail. This was a really long process of animation, and i don't think i helped myself with the length i wanted the animation.
In terms of design, it wasn't a concious decision to go with the 'sperm-looking' tail. But this is what came about, and was easier to draw. I also wanted to concentrate on 'solid drawing', though found that the most challenging part of the animation, as the tail changed in size quite a few times. (though i think it's allowed to...i like the full stretch on it sometimes).
This is the first test.

This is a little extra research with a squirrel tail, something with a little more thickness...just to exaggerate the tail movement.