Rare penguins are not only players that are rare, but also players that care about rarity. Betas yield the rarest item — the beta hat — but are only considered part of this group if they only wear old items and care about rarity.
An item, whether it be a pin, background, clothing, furniture, or an igloo, shan't be considered rare until it is four years old. Most users quit the game after only a couple of years, so these items become quite uncommon.
Club Penguin has stated that all items except for pins and party hats will return. Some items that were made specifically for a one-time party may not return, however. For example, it is unknown whether certain takeover parties will ever return. This may mean that the items won't be brought back again.