Nicknamed "the Kitten" at St. Louis for his resemblance to Harry "the Cat" Brecheen...
I was told over the years that he had that nickname because of his cat-like quickness on the mound! He was the greatest defensive pitcher in the league.
He never let a ball get back up through the middle for a hit.
You may be right about what you said about Brecheen, but Bob Prince (the great Pirate announcer) named him 'the kitten' in Pittsburgh because of his incredible quickness on the mound. Maybe he had that name for various reasons, though.
;-)
Resemblance in that they were both outstanding at fielding hard hit balls back to the mound. Two of the best fielding pitchers ever to play the game. Breacheen was a big guy who they said had the reflexes of a cat. Haddix was a small guy, hence the Kitten.