Wikipedia
The galgo (or yangjang-go or yang-go) is a traditional Korean drum. The drum has an hourglass-shaped wooden body and two drum heads of identical diameter, similar to the janggu. Compared to the janggu, the galgo is fitted with a sound-adjusting funnel different from that of the janggu. The galgo also uses thinner drum skins and is struck with two bamboo sticks called chae but mallet shaped gungchae is not used. The pitch of the drum can be controlled on both sides of the drum rather than only one pitch per-side for the janggu. This instrument is popular after the rule of King Yeongjo in the Joseon dynasties, and is always mentioned in the Jinyeoneuigwe (historical records of the palace). However, today, the galgo has fallen out of use by traditional music performers and if one wishes to see this instrument, normally it will be in The National Center for Korean Traditional Performing Arts.
The Chinese jiegu, adopted from the Central Asian region of Kucha during the Tang Dynasty, is the model from which the galgo is derived. The Japanese kakko, which is used in gagaku music, is also derived from the jiegu.