So, I've mentioned Parasitic Trio before, and that story is going to culminate in a week the protagonists spend at the Battle Frontier in Sinnoh.
Regarding the Battle Hall, I've read on Bulbapedia that 1) only one Pokemon can be used for a winning streak, and 2) only two of the same Pokemon are allowed in the double-battle challenge.
I sorta get the former of those, since the whole point is to challenge you to face all types with just one Pokemon (also the reason you can only select the same type ten times). The latter I'm assuming for exactly the same reason, but it did get me thinking of having my version allow for paired Pokemon. (How the Hall in general would work is a bit complicated to describe here.)
So, here's the real topic at hand:
Some Pokemon pairs, even if they're present in both primary versions of a given game (albeit at different encounter rates), are pretty obvious to spot. For example, Caterpie and Weedle are both caterpillar Pokemon that evolve into cocoons at level 7, and then one more time at level 10. Volbeat and Illumise could not possibly be any more similar without practically being the same Pokemon; you can get an egg of either regardless of which one is male or female when you leave them at a daycare, and that is also true with Tauros and Miltank (gender-exclusive cattle Pokemon, despite debuting a generation apart). Some Pokemon follow a common pattern, such as Oddish and Bellsprout (three evolution stages, third evoked w/ a Leaf Stone), Zangoose and Seviper (co-introduced as sworn enemies), or Vullaby and Rufflet (gender-exclusive baby bird Pokemon w/ two evolution stages).
Others, however, seem rather arbitrary. For example, Meowth and Mankey seem to have nothing in common with eachother, neither do Ekans and Sandshrew, and all Growlithe and Vulpix have is a stone-induced evolution. This would also be true with Electabuzz and Magmar if they weren't Pokedexed right next to eachother, preceded directly by the Ice-type Jynx, both assigned (along w/ Jynx) a pre-evolved form in Gen II, and given item-and-trade-induced evolutions in Gen IV. Sableye and Mawile seem to have nothing in common either, save for Pokedex adjacency and Mega Evolutions introduced in Gen VI (not even in the same game). Some Pokemon are paired only according to specific parts of the franchise, such as Scyther and Electabuzz in the anime episode Showdown at Dark City.
Might make a more comprehensive list if the need arises, but can anyone make heads or tails here otherwise?