The Concise Oxford Dictionary of Proverbs gives these early citations:
1655, J Bramhall, Defence of True Liberty and Human Actions:
“It is the last feather may be said to break an Horses back”
1793 in Publications of Colonial Society of Massachusetts:
“It is certainly true that the last feather will sink the camel”
1848, Dickens, Dombey and Son:
“As the last straw breaks the laden camel’s back…”
It goes on to cite several later examples using “straw” and “camel”, so it may well be that the huge popularity of Dickens’s work helped to fix the saying in that form, just as he seems single-handedly to have introduced “give the cold shoulder”. Or perhaps the saying was already stabilising in that form.
If the animal cited was indeed originally a horse, clearly there’s no need to look for an Arabic origin, but possibly there has been some Arabic influence, or why drag camels into it? Perhaps some parallel saying from the Thousand and One Nights, which I believe were first published in French and English translation in 1704 and 1706 respectively?