Roy Thomas confirmed he was the sole writer of this story. Roy Thomas, Scott Shaw, and Gerry Conway credited as co-creators. Per Roy Thomas, Gerry Conway had the original idea for a funny animal version of the Justice League of America which became later "Just'a Lotta Animals" in Captain Carrot and His Amazing Zoo Crew! (DC, 1982 series) #14. Scott Shaw did the art of the Zoo Crew portion of the story and created Pig-Iron. All other characters were created by Roy Thomas and Dann Thomas (all per email exchange, October 14, 2014).
Klein letters credits from http://kleinletters.com/.
[untitled] Letters Page on Interior Page(s)
1
L-950
Includes letters from Richard Locuson, Andrew MacLaney, Linda Mann and Michael Burkhalter.