Flickr user Monica Wagner turned an old computer monitor into a bed for her dog, Zelda. At the link, you can view fourteen images of it. Sometimes the cat takes over.
That looks so much easier to do than converting a monitor into a fishtank. Still, both of them are pretty neat, especially when old iMac G3 monitors are turned into betta tanks (air stone and everything! the depth of the monitor makes it really easy to fit everything in).
I've also seen an iMac turned into a house for a dwarf hamster, which was amazing and awesome.
while it's a really cool idea, please be careful if you decide to do this yourself. there are very high voltages within an old monitor like that. EVEN WHILE UNPLUGGED. There are electronic components called electrolytic capacitors. They hold a charge even after the monitor is turned off and unplugged. so, be careful! you could get a shock up to several hundred volts.
I've also seen an iMac turned into a house for a dwarf hamster, which was amazing and awesome.