Original Description, pre-debug rom leak
When running through the assembly of the game, it was discovered that there are subroutines that will give you a set part of Pokemon.
It is most likely that these Pokemon the game fills your party with, are Pokemon the debuggers used to run through the game,
to test whether the game is working as intended.
Thanks to @jojo12100 and @BlackShark's help with the codes, to obtain these Pokemon!
Update, post-debug rom leak
Since the debug roms for Blue and Yellow is (at time of writing) now in the wild, the debug Pokémon from those were added.
It is important to note that OT/TID is set upon continuing the game via debug (as opposed to continuing normally via Title Screen).
So while the subroutine is the same, it uses the base save's OT/TID.
In any case, debug roms will not be distributed. Do not ask for them!