Who's This? The lightning bolt on page 31 of Who's Who vol.XXIV.
How you could have heard of it: The Unimaginable returned in 1993 to fight a Green Lantern and Valor in the latter's series (issues 8-10), and then as an important feature of the Supermen of America mini-series (2000). Jeff Lemire has said he was referencing, but not naming or truly featuring, the Unimaginable in the first arc of his Justice League United (2014). Perhaps that reference will be more obvious once we look at an original Unimaginable story?
Example story: Justice League of America #42 (1966) "Metamorpho Says -- No!" by Gardner Fox, Mike Sekowsky and Bernard Sachs
Does the Unimaginable have potential? Well, he's too nasty to be a nuisance à la Bat-Mite or Mopee. And too powerful/haphazard to be a viable long term villain (though a lot of Gardner Fox creations are like that). He reminds me of the Weird in some ways, and could work as a misguided "hero" if he were given humanoid form. I dunno. I guess not.
Who's Next? It's been a long road through Who's Who #24. The next issue has fewer obscure entries, but I can think of 2 or 3. Can you?