Who's This? Who's Who vol.VII, page 7. The sevens have it. Check out this swashbuckling frontier hero:
How you could have heard of him: He only resurfaced during the Crisis on Infinite Earths, in that series, or guest-starring in All-Star Squadron's Crisis tie-in, or in its epilogue, History of the DC Universe.
Example story: All-Star Western #60 (1951)
Now he has returned as a ghost, somehow, and he wants to extort the rest of the cash from the people. What's Don Caballero think of this?
Alas, Don Caballero was quickly replaced by other features (Strong Bow, I think), perhaps because his strip was only marginally "western". He never even got a real name or an origin story, and leaves the DC Universe's rapier fans with a decided void Black Pirate can't fill alone. But that's the Inigo Montoya fan in me speaking...
Who else? I was thinking of lesser villains like the Dragon King or the Dummy, but they both appeared quite close to their Who's Who entries in All-Star Squadron, or one largely forgotten like the Duke of Deception, but I think there's a combination of lesser AND forgotten I can make a meal of next.