ToS #15 brought us the first appearances of Goom…The Thing from Planet X, who would never be more than a minor memory, but his kid, Googam, who appeared issue #17, would go on to appear a few more times.
Googam ends his first appearance sinking into quicksand…
Also in #17, we met Vandoom.
Then in #18, we were introduced to Kraa the Unhuman.
All of these characters, to varying degrees, would appear at least one more time in actual 616 books.