Magic items range in rarity from Common to Artifact. However, I can't find any mundane items that go beyond Common.
Are there any non-magical items of Uncommon rarity or higher? If so, a list/database would be great. If not, is this an intentional design decision? Could a DM create an uncommon mundane item under RAW?
The Dungeon Master’s Guide explains:
Each magic item has a rarity: common, uncommon, rare, very rare, or legendary. [...]
Rarity provides a rough measure of an item’s power relative to other magic items. Each rarity corresponds to character level, as shown in the Magic Item Rarity table.
In contrast, mundane equipment is categorized instead by its gold value, as found in the various equipment tables in the Player’s Handbook. Mundane items are so common that an adventurer could reasonably expect any mundane item imaginable to be available for purchase in any sufficiently large town or city, as the Player’s Handbook explains:
The marketplace of a large city teems with buyers and sellers of many sorts: dwarf smiths and elf woodcarvers, halfling farmers and gnome jewelers, not to mention humans of every shape, size, and color drawn from a spectrum of nations and cultures. In the largest cities, almost anything imaginable is offered for sale, from exotic spices and luxurious clothing to wicker baskets and practical swords.
Magic items generally have substantial effects with direct mechanical implications in play, especially as they become more rare. Rarity is a way to give guidance to DMs about which specific magic items, and how many, players "should" have to keep them from being overpowered. It doesn't matter how many Cloaks of Billowing PCs have at any level, but it would be more consequential if they started finding +3 weapons or the Rod of Orcus at level 2.
Mundane items don't usually have the same sorts of in-game impacts, and so there is less reason for them to have a hierarchy like magic items do. It doesn't matter very much, in game-mechanical terms, if players have a horse or not, and so it doesn't matter very much if horses are common or not. If such a thing is relevant to the plot, the DM can adjust mundane items' rarity as needed. But a published table of rarity won't help for that sort of case-by-case concern.
Since mundane items aren't so important for mechanical balance, there isn't much point in assigning rarities to them. Gold value can restrict availability of mundane items to players, but that's not related to their rarity (depending on the specific D&D setting, for example, seafaring ships might be common, but are still expensive to purchase).
That said, if you want to assign rarity to mundane items I don't see any reason that you can't-- but you'll have to determine the rarity of each item, and game mechanics that use that rarity. That'll be homebrew, not RAW, but outside of the Adventurer's League that doesn't seem like it would be an issue.
My goal was to start my PCs at level 3, assuming they had found/purchased a magic item or two but nothing super rare. I used gold values to limit the power of the non-magical items. I didn't find a good way of doing this (I wanted to limit them to two inexpensive items, but I can't balance that using gold values since magical weapons don't have those.)
– mgiuffrida Sep 08 '20 at 17:46
Since I was starting my players at 3rd level, I wanted a way to provide them with more starting equipment than a 1st level character while still remaining plausible. Since I've learned there's no mechanic for item rarity, I'll try to use gold value instead... maybe X gold and 1 uncommon magic item?
– mgiuffrida Sep 21 '20 at 22:40