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Octillery makes sense, it's just using non-straightforward logic. It's a gun that evolves into a tank/cannon. And the other ti-in that makes it really makes it brilliant is that remora actually have suction cups, that's how they attach to the underside of manta rays. It makes sense that a gun with suction cups would evolve into a bigger gun with more suction cups.
Clamperl also makes sense when it evolves, it's not random. Gorebyss has the clamshell bra, that's part of why it evolves from a clam. Clamperl is an eel egg hiding inside a clam shell for safety. Huntail's mouth is analogous to the a giant hyper-aggressive clam mouth, like if you took Clamperl's shell and put really angry googly eyes on it and it started going nom nom nom nom.
