Thanks.
Pardon that, should've elaborated that four-legged Pokemon creatures could work, so long as they were under Trainers (I could easily see a Trainer having something like other Pkmn Starters such as Bayleef or *gasp* Mudkip, or even an Absol or Heatran, but
never see them fly solo).
Crawling/armless Pkmn are very limited, and rarely are any so supported that they could be made solo like Mewtwo, Jiggs or even Pichu were. All normally depends on popularity, and/or how versatile the Pokemon are (as you mentioned earlier, humanoid Pokemon almost always = A-OK solo Pkmn).
For point of Mewtwo being left in the dust, that's a sad truth. However, I hold out some hope as... he's still a very popular/well-known Pokemon (especially amongst those who grew up with the 1st Generation); First Movie status; and being a rather well marketable Pokemon.
For his appearances... Actually, he's made a few modern comebacks.
His First Movie appearance was in 1998. He appeared in the anime a couple of times, whether in episodes as cameos or (the more frequent) openers... He's appeared semi-recently in the popular Pokemon manga (and "accidentally" kills off the major characters of the series). He's had his "Mewtwo Returns" special... He appeared in the "Mastermind of Mirage Pokemon" special as the lead Poke-villain (was released WITH the Lucario movie as an added bonus), the FireRed/LeafGreen games, the Pokemon Mystery Dungeon games... the Trading Card Game recently released two cards for him, exclusively 1 for Japan and the other for America... Then he also has a shot at being in the new Pokemon HeartGold/SoulSilver remakes in his beloved Cerulean Cave.
Now take someone like Geno, who only made 1 full-game playable appearance back in 1996, and a semi-recent cameo in the Mario & Luigi game... yet his fanbase exploded into motion when it was learned that he had a chance of returning. If a 1-game 1-cameo character can have THAT much support, who's to say someone who's been around just as long cannot gain just as much? (And given he's appeared in a Smash title period gives him extra credit-reputation to reach playable status.)
Hmm. Probably went a little overboard there... :3
As usual, Mewtwo and the Fallen, return to us in Smash Bros. 4. May this inching pipe dream come to pass.