What an interesting duscussion!
First of all the dolphin on the photo Radu send, does have flakes! So is something like a hybrid creature, maybe. The upper part of its mouth goes upward, and this resembles the figure on the heads of the yataghans.
See the first photo i attach with many heads from different makers, they all have this characteristic. So, in Radus photo it is a dolphin with some elements from a serpent or a sea dragon, or what somebody would imagine a creature on the sea should look like.
I believe that the end on scabbards is a basically a dragon, but a dragon that in its appearance resemples a sea creature and hence has some elements from a dolphin.
Basically is a dragon because in other parts of scabbards you can see it there, fighting, eating animals etc, therefore i wouldnt think the artist would have always a dragon on the scabbardd motifs and a dolphin only at the end (see photos) You can also see in a blade a human head figure being among 2 dragons that threaten him from both sides.
Also see in one of the photos i attach how perfect a sea dragon is made on top of the scabbard, like it is half emerged in the water! (each photo is from a different scabbard)
In medieval maps, some of the "sea dragons" are actually whales, but there are also creatures that are pure dragons.
Basicaly the lack of scientific information and photograps allowed people to imagine different posibilities for the dangers of the sea.
Regarding the symbolism, i would say that this could be firstly a "protective" symbol against the sea dragons and the dangers of the sea.
I say this because dragon in the scabbards iconography is the evil enemy that attacks the city, eats the sheep outside and fights with the lion that protects the city.
On the blades, same way 2 dragons threaten the man, but the steel has the magic power to fight evil! So basicaly this is a protective image to "exorcist" the evil.
Finally it is nice to have a ragon or snake head coming out of your belt. It reminds to everyone that there is a danger there and somebody can bite them!