[E] UPDATE ON FEB. 3, 2009. This may not be as accurate as it once was, so take it with a grain of salt as it is for now. Maybe I'll fix it later when I'm not lazy. =P
~~~~~~~~~ I came up with a pet popularity tier to help people with trading. It helps to show how easy some pets might be to trade while you might wait weeks to get an offer on your Kronk. Note that this doesn't take into account stats, learnies, costume or name, just the general pet itself.
Here's a visual (http://i42.tinypic.com/21kgtvr.png), but the chart is below.