It’s possible! Pokémon are very adaptable in ways that frequently surprise the research community. For another example, who would have thought that some pokémon will evolve if they know a particular move or if a metal coat and a lot of radiation were applied to them? Pikachu isn’t even the only one that evolves into a different form thanks to its diet, either; sylveon is just an eeveelution that evolves after being fed delicious treats (and, all right, petted and groomed enough too).
The point is, new and fascinating evolutionary requirements for already discovered pokémon are found all the time, and just when you’ve found what you’d think is the most unusual, a new, more interesting one is discovered. So who knows? Perhaps goldeen will evolve into an undiscovered pokémon after eating an entire box of tea biscuits.
As a disclaimer, I am not encouraging anyone to feed their goldeen an entire box of tea biscuits.
I’m surprised you’re adding that disclaimer on your own. —LH
Well, it needed to be said. Start with one tea biscuit, then work your way up. Starting with the whole box at once is far too ambitious for a goldeen. —Bill
Bill… —LH
The Pokédex According to Bill 