While this is just a thread for unlikely Pokemon, I feel I should clarify on what really makes them unlikely (of the ones that seem to make sense, such as Scizor, Blaziken, Deoxys, etc.)
The first subject is Pichu. Pichu was not included just to give G/S a character. Rather, Pichu was added after the main roster been decided (while cutting Dedede in the process) in order to be a clone of Pikachu and serve as an expansion for the Roster alongside other clones.
But why Pichu? Between Pichu and Raichu, Pichu was far more popular, marketable, and added more to the table by being a joke character.
Prior to this decision, Sakurai felt that none of the popular Pokemon from G/S were feasible. At the time, these Pokemon would be among Pichu, Togepi, Marill, Lugia, Heracross, and Scizor, as well as starter trio. Feel free to add to this list if there are others.
I don't quite understand how Heracross and Scizor would be considered "unfeasible", though they may have been excluded for other reasoning, such as the fact they weren't promoted as much as the prior four.
As for the prior four, I assume this would be the reasoning:
Pichu: Just a smaller Pikachu. (Later served it well when it came to adding clones)
Togepi: Doesn't really suit as a fighter.
Marill: Seems too much like a water Pikachu with Jigglypuff's frame.
Lugia: How would it work without being rediculous? (And no, this has nothing to do with size.)
As for the starters, it's apparently a "all or none" deal, as Sakurai stated one idea he had was essentially the Pokemon Trainer concept, but with the Johto starters instead. However, something like that would have taken too much time and resources.
Come Generation 3, there are plenty of Pokemon that are popular enough that would seem feasible. Those that come to mind are Plusle & Minun (speculated to have been planned as Pichu's replacement), Blaziken, Gardevoir, Rayquaza, and Deoxys.
However, like before, P&M suffer from being MORE electric mice with being a tag-team the only thing that sets them apart.
Blaziken, while probably being the most ideal choice as a fighter, would likely have to deal with the "all or none" mentality that Charizard was dealt in Brawl. While at this point, it would arguably be perfect, given Mudkip's memetic status, Grovyle's overall acknowledgements (such as being a main character in Mystery Dungeon Time/Darkness/Sky), and the female Hoenn Trainer's popularity, would it be worth the cost of the Trainer we already have, or worth the cost of having TWO Trainers with a similar gimmick?
Gardevoir, like Heracross and Scizor, wasn't given much promotion and seemed very "cultish" in terms of fanbase.
Rayquaza is like Lugia in that it'd be hard to make work as a character without being rediculous (and again, size is not the issue here, considering that it is MUCH smaller in the 3D Gamecube/Wii games).
Deoxys, while seemingly good in all grounds; popularity, promotion, and feasiblity, it's very likely that its ability to shift different forms deemed it unfeasible to Sakurai.
Then came the 4th Generation with pretty much the perfect candidate in Lucario.
Popular? Since its movie, Lucario has become a rather big name in Pokemon. Even being among the most requested Pokemon for Brawl, a great feat considering how short it had existed compared to big names such as Pikachu, Charizard, and Mewtwo.
Promoted? Dear Arceus, yes. Well before the debut of the Generation it came out in, Lucario not only had its own movie, but it also had references in other pre-Sinnoh games such as the original Mystery Dungeon, where Lucario was apparently the greatest rescue leader of all time, the highest rank you could get was the "Lucario Rank", and achieving said rank gave you a statue of Lucario as well. And during Diamond and Pearl's promotion, Lucario was one of the more promoted Pokemon as well. Even during the time of the 4th Generation, and now with the 5th, it was/is being promoted well, but that's beside the point.
Feasible? It's a martial arts humanoid jackal that can use a mystical energy. Different from the rest of the cast, but not bordering on rediculous or difficult to incorporate.
Now we come to the 5th Generation. I honestly don't see another Lucario. Zoroark tried to be one, but it has limited popularity at this point, not promoted much anymore, and its signature Illusion power is seemingly difficult to incorporate properly.
Sad thing is, this is the closest we have to a "surefire" 5th Generation representative; which makes a 5th Generation Pokemon unlikely to me.
Hope this mini-rant was enlightening. It was just something I needed to get off my chest for quite a while; it has no negative bearings to this thread.