>>17719162When Nickelodeon walked in, all the art for the pets was "converted" to the new poses so that they could wear clothes. However, when that first happened, some select color/species combos were allowed to remain "unconverted" and keep the old art style. They can't wear clothes, but they look really good. You can't get unconverted pets anymore, so they're pretty rare and trade high.
Top one is the "UC" pet.