You can find the banner dogs by clicking on the banner that you find at teh bottom of the page. You can also find items aside from tokens.
There could be plenty of reasons for the demand, howevere the I think it is mostly because a lot of people think they are cute, they turn out to be quite rare or its because they have a unique lineart.
While there si a limit to how many banners you can click (250) you can find the banner pets/items no matter how many banners you have clicked. I found teh husky on my first banner for example, but I haven't found the other, so its just a matter of luck.
You can see how many banners you've clicked here;
http://www.chickensmoothie.com/halloween/?howmany=1