why can't you do it? why can't you set your monkey free?

monkey, n.
An arboreal animal which makes itself at home in genealogical trees.
(From Ambrose Bierce, The Devil's Dictionary)

Personally, I wouldn't have a pet monkey. They're primates, like us -- no wonder they tend to lash out unpredictably. It's like having one of my cousins for a pet. No offense if you do, though, or have one of those cool service capuchins. Here's an interesting historical tale from Answers.com's page on monkeys:

When the British first began to explore Africa, young monkeys were often
captured to provide entertainment during long voyages. Some were later
transferred to domestic zoos, and in fact many modern captive monkeys in the UK
are descended from individuals captured during the Napoleonic and Victorian
eras. Kent still to this day has the largest population of monkeys in the UK.
According to legend, one of the early British captive monkeys was lost at sea
and washed up ashore near Hartlepool, England, where it was mistaken for a
Frenchman and hanged. The people of Hartlepool have since borne the
nickname "monkey hangers."

There's also an entry on monkey-hangers.