A flippy flappy fish.  

Vaguely sculpted towards the look of a sucker fish but refined by the school of hard-knock toys / destructive testing. 

Further versions may arise depending on interest/survival.

Update V2:

Added flappy Tail.  So… Flippy is now 25% more flappy!  (also improved joint structure)

Technical Aside:

  This was done in Blender E2E (which I enjoy), I have done a handful of articulated / PiP setups in Fusion, but never in Blender and this was a first cut from idea to toy.   I learned a lot.  You'll see 3 distinct types of joint with slightly different design choices.  This was deliberate and I was pushing for minimum print size at max strength.  I didn't love any of the joints, but it's time to move on.

Method:

Take a torus, cut it, grab the edge loop, scale it (with linear fall-off area effect) to make it ramped for printability.   I think if you counted triangles on the edge loop you could make a precisely compatible torus so default bridge edge loops would work well.  I did not count polygons and relied and scripts and sculpting.