A UC pet is a pet that is not customizable. Once customization came out, TNT had to convert all the pets in similar poses so that they can wear clothes. So a UC is a pet that is still in the old pose. They usually look way better than the new poses TNT put them in. Once a pet is converted it cannot go back to a UC. Hence why UCs are valuable.
HB = highest bid so far; AB = autobid (what the trader is looking for)