Speaking of Kecleon. It's one of the last Gen 2 3 pokemon to not get anything to help its sorry state. Most single stages now have evolutions, and Delibird at least has a paradox. Kecleon is just out here kekking.
The slight inaccuracy of Kecleon's generationality aside, this observation is actually quite accurate. Most of the single-stage Pokémon from the early generations have received evolutions, pre-evolutions, alternate forms, Mega Evolutions, G-Max forms or various other stuff through the years, to the point where there are quite few left. I just went through the whole list of "untouched" Pokémon from the early generations on the OI Discord earlier today, and there are surprisingly few single-stage 'mons left that haven't seen
some sort of upgrade over the years.
In Gen I, there's just Ditto. As it is a Pokémon designed around the gimmick of transforming into an opponent, I don't think it is likely to ever receive anything.
Gen II's list is only slightly longer:
- Shuckle
- Skarmory
- Miltank
- Smeargle
- Unown
Unown is one of those "gimmick 'mons" in the vein of Ditto, where it's reasonable to assume that it will never get anything. Smeargle could arguably be counted there as well. Shuckle slightly less so. Miltank could be seen as a counterpart to Tauros, which just received its upgrade as the last and final Gen I single-stage Pokémon eligible for it. Skarmory is as regular a single-stage as they come, and it's a mystery why it hasn't even received a pre-evolution like its counterpart Mantine did.
As for Gen III, there are quite a few more. However, note that several of them were created as counterparts to take advantage of the new gimmick of the generation, double battles, and so should be considered in pairs. Hence:
- Plusle/Minun
- Volbeat/Illumise
- Torkoal
- Spinda
- Zangoose
- Seviper
- Lunatone/Solrock
- Castform
- Kecleon
- Tropius
- Relicanth
- Luvdisc
I think the pairs are unlikely to receive any upgrades any time soon. If they do, they will get them together. It's a shame, though, because they are rather weak Pokémon on their own, and they don't have much synergy even when paired up as they share all weaknesses and STABs. Zangoose and Seviper are "canonical" counterparts, but mechanically they don't have much to do with each other and could be regarded separately. Still not sure if Game Freak would be willing to do so, but if Tauros can be considered separate from Miltank, then maybe. Spinda and Castform are "gimmick Pokémon" and unlikely to see any additions (one can dream, though). Kecleon is a bit of an edge case, with type change as its gimmick, but it is slightly undermined by the distribution of Protean and later Libero to other Pokémon. It still has Color Change unique to it, but that's an ability the opponent can exploit much easier than the user can.
That leaves Torkoal, Zangoose, Seviper, Tropius, Relicanth, and Luvdisc.
Gen IV is in a similar boat to Gen II. There are so few "native" Gen IV Pokémon that there were never that many single-stage families to begin with, so even though most of them were never "upgraded", there are still few enough to list:
- Pachirisu
- Chatot
- Spiritomb
- Carnivine
The first is a Pikaclone, the second a gimmick 'mon, the third arguably a gimmick too, the fourth one is as ordinary as they come.
Gen V, similar story. The single-stages were few to begin with, and while they have barely gotten fewer over time, the list is still short enough to bring up in its entirety:
- Throh (counterpart to Sawk)
- Sawk (counterpart to Throh)
- Maractus
- Sigilyph
- Emolga (Pikaclone)
- Alomomola
- Cryogonal
- Druddigon
- Bouffalant
- Heatmor
- Durant
Not going to bother with gens 6-8. But yeah, early-gen single-stages that haven't seen an evolution or some sort of alternate form (and are eligible to receive one) are becoming quite rare nowadays. The list keeps getting shorter for every generation too. Or longer, I guess, if you consider that many of those forms were only available in past generations.