In the year 2020, a Japanese debug version of Pokémon FireRed and LeafGreen was leaked.
It contained various debug functions, and with them, debug Pokémon can be found.
Sub-routines previously found (that held Pokémon) were mainly: a Bulbasaur egg and a full box of Level 1 Rattata.
(Important to note, that this Rattata that fills up every space you have is actually just a clone, so one copy is enough)
Update: an English debug version was also leaked around a relatively close period to said original Japanese leak.
From the debug menu at the title screen, the player was able to effectively load a predetermined save file,
and such a save contains various Pokémon. However, it appears it was not properly localized, and the trainer name is actually in Japanese.
The Japanese debug also has this feature, and Pokémon from within were subsequently added.
[We will not distribute the Debug ROMs, and it is against our rules to ask for it]