Basically somebody found one of my videos
(https://www.youtube.com/watch?v=_yBGAVUm0dk) and applied it to Japanese Yellow.
What the post is saying basically is if you catch (0xC2) (possible with Trainer escape glitch and a Special stat of 194) and do not have Rapidash set as seen in the Pokédex, its Pokédex entry will appear. Its category will be based on the second character of Pokémon 17 in the Pokémon storage system (beginning at DEC5) and the data that follows.
This is what you get for the quoted example.
裏 means "secret trick" but is actually a cultural phrase. In this case is now the 'secret trick' Pokémon and has the word "" (bug as in a software bug) in the main text of its Pokédex entry.
According to my research for (hex:F9) in English Yellow, the source is F403 (copy of D403) and using the 08 text command character five bytes after the category name's first 0x50 byte could execute arbitrary code.
However, I don't know if the poster was ever able to execute arbitrary code, and placing the 08 five bytes after the category name's 0x50 for doesn't want to work for me for some reason (in fact the main entry text box text occupies this region but it ignored the text command character).
I tested this in v1.0 so there is a chance it could work differently in v1.1, v1.2 or v1.3. However the 2ch poster said it works on Virtual Console Yellow which could be v1.1+(?)