Journalist David Goldenberg of Five Thirty Eight noted the animals most likely used to represent letters in a sample of 50 children's ABC books from 1820 to 2013. He notes that Zebra was used almost exclusively for the letter "Z". But note the letter "X". So few words begin with "X" that it was most often totally omitted from alphabet books or as Goldenberg says used by authors, "lamely trotting out a fox or an ox and pointing out its last letter." The modern trend seems to be using scientific words such as Xiphias for swordfish.
Shown these results, one parent mentioned that Xenops, a genus of ovenbirds, was used in at least two of her son's items (books or toys) and was surprised that "D" for dolphin was not higher in the ranking. But I guess it's hard to top Dog and Duck. And Dr. Susess's ABC Book, for the letter D, dreams up a Duck-Dog!