From Snopes.com,

"Bonsai Kitten" is a site that purports to be "dedicated to preserving the long lost [Oriental] art of body modification in housepets" (by raising them inside jars so that they remain small even when mature and their bodies take on the contours of the vessel used). It's actually a bit of fictional humor put up just before the end of the year 2000 by some MIT grad students to satirize "the human belief of nature as commodity " and to "punish the hypocritical and easily offended by upsetting them, and to amuse those who understand ."