The Harlequin Crest is a quest reward item which can only be obtained in Single Player.
It is a gift given by Ogden the innkeeper as a reward for returning the banner in the Ogden's Sign quest. Ogden said that the Cap belonged to a magician who stayed in the inn some time ago and forgot it when he/she left.
The cap's name is familiar to most players since it was reused for the similarly-themed Unique Shako in Diablo II: Lord of Destruction.
Stats[]

Harlequin Crest
Cap
- +2 to all Attributes
- Armor Class: -3
- -1 Damage from Enemies
- +7 Hit Points
- +7 Mana
- Requirements: None
- Durability: 15
- Qlvl: N/A
- Availability: Quest item, Single Player only
Diablo III[]

During The Darkening of Tristram, this item can be obtained in Diablo III as a Magic helm. It's a nod to the quest Ogden's Sign from Diablo I.
This time, it simply drops from Snotspill at Level 4.
"This cap was left in one of the rooms by a magician that stayed here some time ago. Perhaps it may be of some value to you." — Ogden