Origin of "piggyback?"

The word "piggyback," or "to piggyback" means to carry someone on your back.

What is its origin and why is it a pig?

According to Word Wide Words the expression is a misspell of pick-pack which happened in the 19th century:

Piggyback:

• It started out in the sixteenth century as pick pack, carrying something on the back or shoulders. Pick is a medieval version of pitch, so it meant a load that was pitched on to a person’s back for carrying. A little later, pickpack meant a ride on somebody’s shoulders.
• After that, matters began to get muddled. Pack was changed into back through the obvious associations. Then it became pick-a-back. Finally, the pigs came along, in the nineteenth century, by a confusion between pick and pig, an obvious-enough change, not least because pick made no more sense to people in the word in those days than it does today. Piggy-back came along later in the century, with piggyback a modern loss of the hyphen.

• We’re not sure in what country the pigs were introduced — some writers say it was in north America, others in Britain. There’s lots of evidence from English regional dialects of pig being part of the phrase by the early to middle part of the nineteenth century, which suggests it may originally have been British. Pig-a-back is known from the US no later than the 1860s but from Britain rather earlier — it appears in The Life of Beau Brummell, published in London in 1844, and in A Dialogue in the Devonshire Dialect of 1838 whose glossary explains, “Pig-a-back, said of schoolboys that ride on one another’s backs, straddling, as an Irishman would carry a pig.”

• The earliest cases of piggy-back are from the US in the 1880s, though cases came along soon afterwards in Britain (the OED has a US citation dated 1843, but as this is in a comic description of a riot interrupting a wedding and refers to men actually carrying pigs, it looks like wordplay on pick-a-back). I’d guess the same processes of change were going on in both countries more or less at the same time and pace.

It is a corruption of the phrase Pick-a-back. It has nothing to do with porcine animals.

The phrase simply means to carry something on your back, or carry a pack.
Over time, piggyback has come to mean carrying person on your shoulders.

Could it have anything to do with the Chinese book Journey to the West where a pig man carries his wife on his back? Or just coincidence? One of the great novels of Chinese literature, written in the 16th century but probably not known well in the west until much later, with the first English translation in 1942.