Ultra Rares are items that are extremely rare drops, designed to be acquirable only through chance (or loads of meat). Jick has done an incredibly good job of keeping them virtually inaccessible to farmers (or at least as inaccessible as to occasional visitors), and the mechanic by which they occur is unknown. There has been much debate over the best strategy to acquire one of these. As yet, the only way which seems to have a good chance of paying off in the long run is to save your meat.
All ultra rares pulverize into ultimate wads.
List of Ultra Rares