It was Ho-Oh. It had already been designed at this point.
jRed/jGreen, jBlue, usRed/usBlue actually had 190 Pokemon programmed into it. However, 39 of the Pokemon couldn't be implemented fully into the game because of the storage limitations of the cartridge. These Pokemon can be generated and fought as different MissingNo.s by forcing the game to give you an encounter for the corresponding Pokemon's index number.
Eg., Ho-oh is actually index number 34. Victreebel is actually number 190. Rhydon is number 1.
If you can successfully transfer them into the Gen II games, they turn into the Pokemon they were supposed to be. If you generate and catch a Pokemon with an index number above 191, you can't transfer it into Gen II. It's just garbage data.