>>49850677
It's convergent like Ichthyosaurs and Dolphins because efficiency of function. You nee something that can envelope as a vessel for the member. You need something to close when not in use to stay wet and not contaminated by the environment. Then the monster generally needs something wider at the end of a tapering tail. You additionally a way to form constriction. That's only going something built like a closing mouth/fingers with multiple parts or round muscles like a sphincter.

That pretty much gives you the shapes you describe. You could have a large flat sheet of slick mucus-coated muscular tissue like an amphibian tail and have it wrap around the member in a sealed environment, but that winds up just being more of a tailjob.

The last form I can think of would be the space covered by some kind of hinged top or flap, like a Cretaceous teapot oyster. That starts to look really weird really quick, but allows you to hide it in about any hard structure. Just imagine some dragon covered in spikes with the biggest one on her tail. Then, in a fight, just about when you're going to get impaled, the top flips up, and instead of a little flag that says "Bang" popping out, BAM! Tailspike Pussy. Bonus, you do cute little art things like have different onahole patterns on the tails, like red rings on a white spike for different markings.