Who's This? Who's Who vol. XIV, page 14, the top half. Can you believe a Teen Titan got only half a page?
How you could have heard of him: Well, despite my thinking of him as an obscure 70s character, he did make a number of appearances in post-Crisis continuity, though Secret Origins Annual #3 made it clear he was never Guardian (the African-American Guardian of the 7 Soldiers of Victory is presumably a tribute to Mal's short time in the role), and that he used the less risible name Herald instead of Hornblower. As Herald, he was a member of Titans West (or equivalent) and saw action in the mid-2000s, and after 52/One Year Later, joins a new Doom Patrol. He's also enjoyed appearances in various cartoon shows.
Example story: Teen Titans vol.1 #45 (1976) by Bob Rozakis, Irv Novick and Vince Colletta
Who else? Well, I think I might revisit the Golden Age next...