As I've been reading much of the early Superboy stories in Adventure and now Superboy, I've been surprised to see that many of the things we thought were newly introduced in the Silver Age actually were reworkings of Golden Age storylines and characters.
For example, consider Titano, the giant ape with Kryptonite vision:
It turns out that in Adventure #219 (December 1955), well before Titano's 1959 debut came Chandu, the gorilla:
What about the Thought Beast from Krypton?
Turns out he was swiped from Superboy #22, (Oct-Nov 1952):
I haven't yet been able to locate a third example, although I'm certainly looking for the metal-eating beast.