The first time I heard "whale" mentioned in the video game space was at a GDC panel around a game called Puzzle Pirates, where the dev noted that the bulk of their in-game purchases were made by a small group of "whale" players. In 2003 purchasable skins, pets, hats, etc were pretty new.

I remember that one and remember being super exasperated about not being able to do anything without paying upfront for it. Ended up switching to Club Penguin instead lol.