Seussville is an ugly and terrifying pocket dimension of the UnUniverse created by Dr. Seuss as a home for his creations that no one wanted to go. The laws of physics don't apply in Seussville, giving it's inhabitants horrific powers. A portal connecting Seussville to our world was created by The Cat in the Hat for use by Seussville's inhabitants, and is kept in a crate. It consists of ugly creatures made by Dr. Seuss, ugly looking buildings, and a pink lake.
Seussville was first created to be a land to hold all of Dr. Seuss' goo. He simply has so much goo, it was threatening to destroy the world. He created this pocket dimension to store all of the goo. After this crisis had been solved, Dr. Seuss took a nap, hoping he would never have to put anything in Seussville again. He decided to ignore the fact that the goo needed to be refrigerated after opening.
Dr. Seuss, being the fool he is, accessed the dimension only a mere three hours later. Dr. Seuss created a monster known as Horton. Fearing this monster might escape and destroy a gas station, he locked it in Seussville. He decided making monsters is fun, and made a whole bunch of different creatures, all of which got locked in Seusville.
By 2003, the Cat in the Hat had created a portal, allowing all of the creatures to escape. Dr. Seuss solved this problem by creating two things to guard this portal. However, the Cat in the Hat offered them candee, so now they work for him. This means anyone can get through the portal, assuming the Cat doesn't pay any attention (and he's too busy harassing some random kids to pay attention to something as unimportant as a portal that can destroy the world).
Seussville was the home to over 5,000 different creatures of various sorts. Some of them are very odd indeed. In fact, one of them even has a mustache.