The entire episode, "Nothing to Fear," could count as this, which features everyone's worst fears coming to life and attacking them. For young minds watching this episode in the dark, it is one of the scariest things you can possibly ever watch. Just wait until you see the vacuum cleaner.
A banana begs Doofus not to eat him. Then it turns into a giant monster going to eat Doofus.
The Golden Death in the 2-part series finale. As you get older after watching this one the Fridge Horror truly sets in. For example, there's the scene in which Scrooge's mansion is covered and everyone inside is transformed into a gold statue. It moves so quick that if you're caught unprepared: one second you could be minding your own business and the next you'll never know anything again. And even if you do manage to see it coming and try to escape, you'll get less and less space to run to since it will cover the entire world, and after touching something that's been changed you'll also be turned to gold within seconds. Even taking to the air would ultimately be fruitless since there's only so long you could stay up before your plane ran out of fuel and forced you to land, sending you into oblivion.
"Hotel Strangeduck" is creepy no doubt, but there wasn't anything in it that sent it into complete nightmare territory... at least up until the point where the shadow of a skeleton comes out and starts strangling Uncle Scrooge.