A couple of books on the subject are:
The Animals' War by Juliet Gardiner.
ISBN-10: 0749951036
ISBN-13: 978-0749951030
and...
The War Horses: The Tragic Fate of a Million Horses Sacrificed in the First World War.
ISBN-10: 0857040847
ISBN-13: 978-0857040848
I have both in my bookcase and they are available in the UK and US on Amazon, but the second book is a bit pricey Stateside.
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