I never knew about this, either. Learn something new every day.
I've been trying to figure out what is happening, but the event script is not behaving normally. The event that occurs the first time you enter the Big Whale activates NPC (a person/object) D8 (at least the mystery of why there's two events loading the Big Whale map is solved). However, when I activate this NPC separately, it doesn't seem to do anything. Also, swapping a character in Baron for the character D8 shows a Dancer, which seems right considering D0-DA are all clerics/dancers/Edward in Toroia.
Furthermore, I checked the location properties for that chocobo forest, and its person/object layout references 5F, which is empty. Switching another chocobo forest to this layout makes the forest empty, not have a black chocobo. Strange, huh? My guess is that there must be code somewhere that switches the person/object layout of the Baron chocobo forest depending on whether character D8 (or perhaps the Big Whale) is active or not. I don't know why, but that's the best I can come up with. I'll keep looking into it and see if I can figure it out.