because that’s how much they used to cost.
At least that’s my understanding.
Guinea-pigs were called guinea-pigs several years before the coin was first minted in 1663.
Besides, that would be a darned expensive guinea-pig. In the days before rampant inflation, a guinea was an awful lot of money, something like $50 in today’s U.S. currency.
Not that I’m particularly married to this theory, but:
a) Can you point to a pre-1663 document that uses the name guinea pig? If so, end of theory. If not?
and
b) While $50 may seem pricey in today’s guinea pig market, isn’t it possible that, back in the day, these critters, being indigenous to the New World, were considered exotic? Is it so absurd to think that an animal which could only be obtained by traveling halfway around the world might fetch such a price when you finally got it home?
Not that I’m particularly married to this theory, but:
a) Can you point to a pre-1663 document that uses the name guinea pig? If so, end of theory. If not?
and
b) While $50 may seem pricey in today’s guinea pig market, isn’t it possible that, back in the day, these critters, being indigenous to the New World, were considered exotic? Is it so absurd to think that an animal which could only be obtained by traveling halfway around the world might fetch such a price when you finally got it home?
Merriam-Webster gives 1664 as the date for the term “guinea pig”, by which time the critters had already long since made their European entrance. Queen Elizabeth reportedly had one, 75 or so years earlier, but then, she had an entire menagerie in the Tower of London, so nothing surprises me.
http://cavyhistory.tripod.com/
Webster’s Deluxe Unabridged (oof) doesn’t give a date, but notes that they were probably brought by ships “plying between England, Guinea, and South America”.
But here’s another possible explanation.
http://www.ceismc.gatech.edu/zooary/zoo/birds_mammals/guinea_pig.html
Here’s what a “cony” is–a pika.
http://www.enchantedlearning.com/subjects/mammals/lagomorphs/Pikaprintout.shtml
It does look a lot like a guinea pig.
P.S. You can print this out and color it at home! 
Purchasing power of 17th century money. Math. Wah! :eek:
http://www.portsdown.demon.co.uk/coin.htm
A guinea was worth 21 shillings, and a shilling was worth 12 pence.
http://www.kipar.org/history_coins.html
So if the price for a gen-yoo-wine South American cavy was a guinea, in say Portsmouth, which was a port facility, and where one would expect the price on incoming novelties to be the lowest, that would have been the equivalent of 252 loaves of bread. Yes? 12 pence in a shilling, 21 shillings in a guinea, bread at a penny a loaf.
So today you can get a decent loaf of bread for $1.50, which would make your nifty South American novelty, just off the boat, worth $378, or about the price of a 25" color TV with a budget VCR.
Which seems pretty steep, I’ll admit, but then again, you can’t buy male and female 25" color TVs, and in six months be in business for yourself selling cute little baby color TVs.
…and market forces, free enterprise, and cavy breeding capabilities being what they are, I wouldn’t have expected the “guinea a head” price to last very long, even a hundred years before they had an actual coin called a “guinea”.
So I think the “cony pig” explanation makes sense, as does the “Guineamen” explanation.