Who's This? One of several Turtle Men on page 18 of Who's Who vol.XXIV. Not sure why Who's Who uses the name "Turtle Man" when the entry says the original was "The Turtle" and admits the second was last seen using "The Turtle" as his nom de guerre.
How you could have heard of them: Once Wally West moved to Keystone City, he had to confront the original Turtle, now a crime boss with an armored henchman called the Turtle Man. The Silver Age Turtle Man was also featured as a fan of the original who had been crippled in an explosion (as told in Flash volume 2 #35, 1990). They appeared a number of times between 1990 and 2005, and occasionally elsewhere.
Example story: Comic Cavalcade #24 (1947) "The Slow-Motion Crimes" by Robert Kanigher, Carmine Infantino and Frank Giacoia (it has been reprinted in The Greatest Flash Stories Ever Told)
Who else? Let's quickly move on to another subject... and to another Earth!