Caught a Lapras on Pokemon Go this morning and was trying to figure out why, years on, that Pokemon still excites me even though it's common as hell in the modern games.
I think it's because Lapras, moreso than any other Pokemon, is so mythologised in the first few gens. Few Pokemon have gotten that much attention outside of their dex entries - it's one of the most lore-heavy species that isn't a legendary.
In Gen I it's obviously given as a gift and its rarity and specialness are emphasised; in Gen II several NPCs talk about the strange roars from deep within the Union Cave, making it a cool mystery that you'll remember and provide incentive to return later in the game. In Gen III Lorelai's Lapras is one of the rare examples of an NPC's Pokemon that gets some actual characterisation: she says it's angry that the Rockets are hurting its friends, and one of the NPCs on Four Island later tells the player that she caught the Lapras years ago in the Icefall Cave when she was very young and has been with it ever since.
Gen IV and V don't do much with it (outside of HGSS doing the same thing), but it remains prominent by dint of being elusive and by being a gift again in Gen VI, and then of course Gen VII gives us the happy news that it's no longer endangered and gives it the spotlight as a ride Pokemon. But that consistent status as a story-relevant mon and the way the games continue to favour it with unique Surfing models and suchlike mean that it's one of the few Pokemon that still feels special and rare.
That's hard to achieve for any of the Gen I species, since almost by design every gen dilutes the rareness and uniqueness of each species. Snorlax would be the obvious comparison to Lapras, but in Gen I Snorlax is just kind of... there. A man on the SS Anne references it as an amazing sight, but it's not discussed in any great detail and the fact that it's roadblocking two parts of Kanto is pretty much unreferenced outside of a couple of missably NPCs idly mentioning it. It's much the same in Gen II; DPP make Snorlax special by making Munchlax so damned rare, but in Gen V it's quite easy to obtain and in Gen VI it's another roadblock (and available way too early IMO). Gen VII makes Munchlax common from the start: it was a gift mon in SM and it's wild on Route 1 in USUM. It's not really that rare or special any more.
But yeah. Lapras still, somehow, manages to excite me whenever I see one. Idk what other Pokemon from the early gens I could say the same of.