8 traits, from Unarmed to Urdefhan.
Unarmed · Unbounded · Uncommon · Undead · Undine · Unique · Unstable · Urdefhan
Trait
An unarmed attack uses your body rather than a manufactured weapon. An unarmed attack isn't a weapon, though it's categorized with weapons for weapon groups, and it might have weapon traits. Since it's part of your body, an unarmed attack can't be Disarmed. It also doesn't take up a hand, though a fist or other grasping appendage generally works like a free-hand weapon.
Core Rulebook, page 283.
Trait
Planes with this trait loop back on themselves when a creature reaches the plane's "edge".
Gamemastery Guide, page 255.
Trait
Something of uncommon rarity requires special training or comes from a particular culture or part of the world. Some character choices give access to uncommon options, and the GM can choose to allow access for anyone.
Less is known about uncommon creatures than common creatures. They typically can't be summoned. The DC of Recall Knowledge checks related to these creature is increased by 2.
— Bestiary, page 344.
Core Rulebook, page 637. Expanded in B1. Reprinted in G&G.
Trait
Once living, these creatures were infused after death with negative energy and soul-corrupting evil magic. When reduced to 0 Hit Points, an undead creature is destroyed. Undead creatures are damaged by positive energy, are healed by negative energy, and don't benefit from healing effects.
Core Rulebook, page 283. Reprinted in B1, B2, SoM.
Trait
Undines are planar scions descended from marids.
Bestiary 2, page 310.
Trait
A rules element with this trait is one-of-a-kind.
A creature with this rarity is one of a kind. The DC of Recall Knowledge checks related to this creature is increased by 10.
— Bestiary, page 345.
Core Rulebook, page 637. Reprinted in G&G.
Trait
Unstable actions rely on experimental functions of your innovation that even you can't fully predict. After an unstable action is used on an innovation, using another one is dangerous.
Guns & Gears, page 236.
Trait
Urdefhans are humanoids who have pacts with daemons, have transparent skin, and drink blood.
Bestiary 2, page 310.