In fairness to this one it's noted that it doesn't have a proper shell anymore. It's just vestigial plate cells now.
& turns out Shellos name in japanese is even meant to reference not having it anymore. The English one is probably ALSO meant to do that (Shell + loss) but "los" doesn't read as "lost", personally...
So with this one I was actually taking my thinking from the fact that Gastrodon and Cloyster are apparently closely related (if you want to believe the Pokedex, which is always a foolhardy thing to do)
Regardless I'm all for being a bit loose with move theming and I don't think you need an actual, literal shell to smash - Torterra barely does, and Barbaracle is a stretch. Fair point on the Shellos naming, but Gastrodon having a vestigial plate is more than enough to justify getting the move even via breeding IMO. Egg moves generally are about giving something a looser, tangential ability it could feasibly have with some outside help, after all.
What? Learning a sleep move doesn't automatically equal learning Dream Eater, none of the mushroom Pokémon get it.
Sure, but Poliwrath has always had a hypnosis theming - much more so than, say, Ninetales, Gulpin, Abra, Exeggutor, or Starmie. It learns Psychic and Psywave and various mind/willpower-focused moves like Mind Reader and Encore, I don't think Dream Eater is at all out of the question for it.
I mean, Entei doesn't get it either. Volcano themeing doesn't automatically get a Pokémon Earth Power.
Literally the point of my post was "it would be fitting for them", so yes I would say Entei should probably learn it too.
Quiver Dance is pretty much restricted to actual butterfly/moth Pokémon or ones that are explicitly based on dancing (Bellossom, regular Lilligant, and Oricorio).
A butterflyfish or vaguely butterfly inspired tiki creature are probably a bit too far. Also Quiver Dance Tapu Lele would be kind of broken.
There are countless examples of a Pokemon getting a move based on word association or theming. Being a butterflyfish didn't stop Lumineon from getting U-Turn, Silver Wind, Signal Beam, Air Slash, Air Cutter, Tailwind, and even Roost in L:A. I've long thought it could do with something more to set it apart from being a generic fish.