So it swings its tail around...? I'm just having trouble visualising Seviper dancing.
Seviper are pretty nimble with their tail:
Also, nothing says that Swords Dance needs to be elaborate. I can imagine Swords Dance for Seviper being it coiling up and wiggling its tail around in the air (like a reverse cobra), contracting its body muscles to force the energy and venom to gather up into its tail.
Serperior can learn Swords Dance too, for the record
It ain't no slouch either:
Honestly I think the animators have a lot of fun animating the long serpent Pokemon because they can make them twist and turn in all sorts of interesting ways due to them pretty much just being a flexible tube.
There are more than 800 Pokémon. You can explain anything you want and it will make sense one way or the other; too much exceptions. Like Rapidash get access to SD because it has horns, or Serperior because its royal, or whatever you want because its blue and there are blue mons that learn that move...
Sure, there are over 800 Pokemon, but they didn't have to go over 800+ Pokemon movesets at the same time.
In Gen I they only had 151 Pokemon.
Gen II only 100 Pokmeon.
Gen III only 135.
etc., etc..
So in that regard, it's perfectly reasonable they assigned each Pokemon their movesets cause they only had to work in 100 batches at a time.
Then, when they make a new move, they probably think "okay, what Pokemon would logically learn this move" and see if they can fit it into an old Pokemon's moveset in someway. Sometimes they fit it in the level-up (maybe as a Level 1/Move Reminder exclusive), sometimes leave it as a TM/TR/Turor, or maybe make it an Egg Move (which could explain why sometimes a Pokemon gets an Egg Move that it has no possible way of learning. This idea also probably works with TMs/TRs and Tutors; after making the list thinking what Pokemon may get it.
Also, region's Pokedex isn't only the new Pokemon. After making a regional dex they probably give older Pokemon's movepools a quick lookover to see if maybe there isn't any changes that need to be made whether to improve upon it or for game balance reasons (last one notable as it explains why Pokemon learnsets change & shuffle levels sometimes).
Most Egg Moves they likely see what Pokemon the individual can breed with and going "hey, this could be an interesting move this Pokemon learns it wouldn't normally". Curious whether they do this with the Field Egg Group or just toss their hand in the air with that one singe Smeargle makes that a moot point.
Most Gen 2 event mons had absurd move distributions
Oh Petal Dance, a high BP grass move?
Grass mons that can learn it from the event: Chikorita
None Grass mons: Psyduck, Cleffa, Igglybuff, Smoochum, Pichu
Like, wut?
I can totally see the baby Pokemon just picking flower petals off of nearby flowers and throwing them in the air like confetti... then use their elemental powers to make them into an attack cause why not.
As for Psyduck... maybe it got confused and thought it was a Grass-type.