Dang, anything with Dream involved sounds awesome. Lugia with Multiscale... Whoa. And there was a Seismic Toss Blissy? If that's so, old-school rare movesets or hard to get Events ought to be rare, and then we'd have to factor in who would want to offer the crazy Pokemon anyway.
Yeah Dream Radar was this 3DS game app and if you inserted a legit Platinum, Diamond, Pearl, HeartGold, or SoulSilver cartridge into the slot after u had beaten Dream Radar then you would unlock a mission in order to capture Giratina, Dialga, Palkia, Ho-Oh, or Lugia respectively (depending on the game u inserted). Ho-Oh and Lugia would have their hidden ability and they are way better than Pressure! :D
Yeah in LeafGreen and FireRed you could teach Chansey or Blissey Seismic Toss from a move tutor, its the single only way to teach them that move (which is arguably their most reliable and best way of dealing damage).
There were also event pokemon from gen 3 that had some moves like Chansey with Wish, and the like.