I recently looked into this myself.
I used the Internet Archive to do, in a similar vain to one man's venture I had seen some years back, a sample of 49 books. From there I gathered each animal listed in the book for each particular letter, and then catalogued whichever one was the most frequently-appearing in said book.
I have tried to avoid 'localised books'; which focus on a particular area or group of animals; and stick to more 'general' books of general animals [mostly mammals, by any rate].
If the letter has no matching object worth including in the book, it does not take a part of the sample.
And if there is more than one animal depicted in the book for a given letter it is counted individually. So although I have sampled 49 books, most of them have more than 49 different animals listed. X had only 48.
And this is the list produced:
ALLIGATOR [ANT]
BEAR [BUTTERFLY]
CAT [CAMEL]
DOG/DUCK [DOLPHIN]
ELEPHANT [EAGLE/EGRET]
FROG [FOX]
GIRAFFE [GORILLA]
HIPPOPOTAMUS [HORSE]
IGUANA [IBIS]
JAGUAR [JELLYFISH]
KANGAROO [KOALA]
LION [LLAMA/LADYBUG]
MONKEY [MOUSE/MOOSE]
NEWT [NIGHTINGALE]
OWL [OCTOPUS]
PENGUIN [PANDA]
QUAIL [QUETZAL]
RABBIT [RHINCEROS]
SNAKE [SEAL/SKUNK/SQUIRREL]
TIGER [TURTLE]
UNICORN [UMBRELLABIRD]
VULTURE [VOLE]
WHALE/WALRUS [WOLF]
X-RAY FISH [XENOPS]
YAK [YELLOWJACKET]
ZEBRA [ZORILLA]
So the most common animal is that which immediately appears; and the second-most is in brackets.
But this is in no way proportional. In the case of 'E', for the fifty-six different animals [and perhaps other objects] listed, Elephant appeared 50 times; so 89% of the 'E' inclusions were elephant. On contrast, Eagle and Egret were only threefold each.
Some letters seemed to have 'clear majority'; with the most common animal appearing 50% of the time of more; namely - G, J, K, L, Q, T, V, Y, and Z.
And of these, Yak was the most commonly-appearing animal proportionately speaking; of the 50 entries for 'Y', Yak appeared 43 times. [89%!] And for the 54 entries under Z, there were 44 Zebras.
Part of me wonders whether period of time would have effect on what animals would be present, but part of me also thinks not ... as even throughout different time periods you can see variety of different animals used. Unicorn for sure being most common; but some writers had the decency to use animals of the physical realm.
For 'X' there is some tendency to avoid the subject - one thing that becomes apparent doing this most of all is that these authors have tendency to copy each other. E is for Elephant because that's how it's always been, Y is for Yak since the time of my Great Grandfather. So there is plenty of X-ray fish in the sea for sure ... and also 12 'non-answers' - amongst them, fox, ox, X-ray, Xavier, or some animal the author made up. And possibly a thirteenth - one book I found listed 'Xoona Moth' as its 'X' animal, but I haven't found much about this particular animal ... outside of another animal alphabet book that is. So it may very well be again a case of someone copying someone else.
n.b. No it wasn't - apparently that is the name of this moth [Owl Moth] used by the Tsonga Tribe of Mozambique ... very well then!
One thing I was also interested in was whether one book had this particular lottery list of 26 animals - to which I found that was not the case. The authors did seem to steer clear of the total average!