Undertale - Gaster, the spoiler.

But Gaster isn't that hidden anymore, after 1.001. That rule shouldn't abide anymore. Also, everybody already knows about Gaster, so screw that rule.

He can appear without hacking.

I'd like to mention the rule is dumb anyway. How is he a spoiler if you'll never encounter him normally? The only reasoning I got is that looking him up will lead you onto hitting other spoiler-y things… AND THOSE THINGS AREN'T SPOILERS HERE!