Have you noticed that in children’s books, Z is almost ALWAYS for zebra? It might be for something else as well, but zebra is nearly inevitably represented. For instance:
Poor Puppy by Nick Bruel (2007)
A to Z by Sandra Boynton (1984, 1995)
A to Z Animals: A Bedtime Story by Danice Baker. Illustrated by Judith Moffatt (2005)
Robert Crowther’s Most Amazing Hide and Seek abc Alphabet Book (1977, 1999)
The Alphabet Book by P.D. Eastman (1974)
Even in non-alphabetical books, the zebra will get its due, such as in The Zoo Book by Jan Pfloog (1967); even the book is in the shape of a zebra.
Now, in English, an X in the first position usually sounds like an Z, such as xylophone. (An exception is a word like X-ray, where the letter sounds like ks.) Maybe, to lessen the burden on the poor zebra, we should attribute to Z some of those X words. Or not.
One of the first jokes I ever learned: what black and white and red all over? A sunburned zebra.